diff --git a/nodejs/src/common/utils/db/utils.ts b/nodejs/src/common/utils/db/utils.ts
index 119b1f0f41dd..b1f0be269196 100644
--- a/nodejs/src/common/utils/db/utils.ts
+++ b/nodejs/src/common/utils/db/utils.ts
@@ -111,15 +111,11 @@ export function personInitialAndUTMProperties(properties: Properties): Propertie
 
     // For the purposes of $initial properties, $os_name is treated as a fallback alias of $os, starting August 2024
     // It's a special case due to _some_ SDKs using $os_name: https://github.com/PostHog/posthog-js-lite/issues/244
+    // normalizeOsAlias fills the event's own $os upstream, so the loop above already lifted $os into $set.
     const osName = properties.$os_name
     if (osName !== undefined && !skipServerHostOs) {
-        if (!('$os' in properties)) {
-            properties.$os = osName
-        }
-        if (!('$os' in $set)) {
-            $set.$os = osName
-        }
-        // Same null rule as above: a null $os_name must not claim $initial_os for good.
+        // Only reachable when $os is null, since the loop skips $initial_os for a null value; the
+        // non-null check on $os_name is the same first-touch rule as above.
         if (osName !== null && !('$initial_os' in $set_once!)) {
             $set_once!.$initial_os = osName
         }
diff --git a/nodejs/src/common/utils/event.test.ts b/nodejs/src/common/utils/event.test.ts
index b3519ad66abe..0c7983fc085a 100644
--- a/nodejs/src/common/utils/event.test.ts
+++ b/nodejs/src/common/utils/event.test.ts
@@ -62,6 +62,69 @@ describe('normalizeEvent()', () => {
         })
     })
 
+    describe('$os_name alias', () => {
+        test.each([
+            {
+                name: 'fills a missing $os',
+                event: 'some_event',
+                properties: { $os_name: 'Windows' },
+                expected: 'Windows',
+            },
+            {
+                name: 'leaves an existing $os alone',
+                event: 'some_event',
+                properties: { $os: 'Mac OS X', $os_name: 'Windows' },
+                expected: 'Mac OS X',
+            },
+            {
+                name: 'carries a null $os_name through to $os',
+                event: 'some_event',
+                properties: { $os_name: null },
+                expected: null,
+            },
+            {
+                name: 'skips a $snapshot event, which skips person property lifting too',
+                event: '$snapshot',
+                properties: { $os_name: 'Windows' },
+                expected: undefined,
+            },
+            {
+                name: 'skips a $performance_event, which skips person property lifting too',
+                event: '$performance_event',
+                properties: { $os_name: 'Windows' },
+                expected: undefined,
+            },
+        ])('$name', ({ event, properties, expected }) => {
+            const normalized = normalizeEvent({ event, distinct_id: 'user1', properties } as any)
+            expect(normalized.properties!['$os']).toBe(expected)
+        })
+
+        it('gives the person $os and drops the $os_name alias', () => {
+            const normalized = normalizeEvent({
+                event: 'some_event',
+                distinct_id: 'user1',
+                properties: { $os_name: 'iOS' },
+            } as any)
+
+            expect(normalized.properties!['$set']).toEqual({ $os: 'iOS' })
+            expect(normalized.properties!['$set_once']).toEqual({ $initial_os: 'iOS' })
+        })
+
+        it('keeps a server host OS out of $os and $initial_os', () => {
+            const normalized = normalizeEvent({
+                event: 'some_event',
+                distinct_id: 'user1',
+                properties: { $is_server: true, $os_name: 'Linux' },
+            } as any)
+
+            expect(normalized.properties!['$os']).toBeUndefined()
+            // The host OS still reaches the person under the $os_name keys: personInitialAndUTMProperties
+            // skips only $os and $os_version for a server event, and $set_once makes $initial_os_name sticky.
+            expect(normalized.properties!['$set']).toEqual({ $os_name: 'Linux' })
+            expect(normalized.properties!['$set_once']).toEqual({ $initial_os_name: 'Linux' })
+        })
+    })
+
     it('combines .properties $set and $set_once with top-level $set and $set_once', () => {
         const event = {
             event: 'some_event',
diff --git a/nodejs/src/common/utils/event.ts b/nodejs/src/common/utils/event.ts
index 21123314f47c..51efc20588ad 100644
--- a/nodejs/src/common/utils/event.ts
+++ b/nodejs/src/common/utils/event.ts
@@ -1,4 +1,4 @@
-import { PluginEvent } from '~/plugin-scaffold'
+import { PluginEvent, Properties } from '~/plugin-scaffold'
 import { ClickHouseEvent, PipelineEvent, PostIngestionEvent, RawClickHouseEvent } from '~/types'
 
 import { personInitialAndUTMProperties, sanitizeString } from './db/utils'
@@ -136,6 +136,30 @@ export function sanitizeEvent<T extends PipelineEvent | PluginEvent>(event: T):
     return event
 }
 
+/**
+ * Fills the event's own $os from the $os_name some SDKs report instead:
+ * https://github.com/PostHog/posthog-js-lite/issues/244.
+ *
+ * This describes the event, not the person. Call it after transformations: the user agent populator
+ * plugin skips an event that already carries $os, so filling $os earlier would stop it deriving
+ * $browser, $device, and the rest.
+ *
+ * Server events are skipped so a server's host OS does not land in the event's own $os. The person
+ * side is handled separately: personInitialAndUTMProperties skips lifting $os and $os_version and
+ * skips folding $os_name into the sticky $initial_os for $is_server events. The host OS still reaches
+ * the person under the raw $os_name and $initial_os_name keys.
+ */
+export function normalizeOsAlias(properties: Properties): void {
+    if (properties['$is_server'] === true) {
+        return
+    }
+
+    const osName = properties['$os_name']
+    if (osName !== undefined && !('$os' in properties)) {
+        properties['$os'] = osName
+    }
+}
+
 /**
  * Full event normalization including person property mapping.
  * This should only be called ONCE per event, after any transformations.
@@ -146,6 +170,7 @@ export function normalizeEvent<T extends PipelineEvent | PluginEvent>(event: T):
     event = sanitizeEvent(event)
 
     if (!['$snapshot', '$performance_event'].includes(event.event)) {
+        normalizeOsAlias(event.properties!)
         event.properties = personInitialAndUTMProperties(event.properties!)
     }
 
diff --git a/nodejs/tests/utils/db/utils.test.ts b/nodejs/tests/utils/db/utils.test.ts
index cc8630ddd937..41c3cd909a69 100644
--- a/nodejs/tests/utils/db/utils.test.ts
+++ b/nodejs/tests/utils/db/utils.test.ts
@@ -101,23 +101,28 @@ describe('personInitialAndUTMProperties()', () => {
             $set: { $current_url: 'https://test.com' },
         })
     })
-    it('treats $os_name as fallback for $os', () => {
-        const propertiesOsNameOnly = { $os_name: 'Android' }
-        expect(personInitialAndUTMProperties(propertiesOsNameOnly)).toEqual({
-            $os: 'Android',
-            $os_name: 'Android',
-            $set_once: { $initial_os: 'Android' },
-            $set: { $os: 'Android' },
-        })
-        // Test that $os takes precedence, with $os_name preserved (although this should not happen in the wild)
-        const propertiesBothOsKeys = { $os: 'Windows', $os_name: 'Android' }
-        expect(personInitialAndUTMProperties(propertiesBothOsKeys)).toEqual({
+    it('keeps $os over $os_name and drops the alias from person properties', () => {
+        // normalizeOsAlias fills the event's own $os upstream, so this function always sees $os
+        // alongside $os_name; the divergent values here never occur in the wild.
+        const properties = { $os: 'Windows', $os_name: 'Android' }
+        expect(personInitialAndUTMProperties(properties)).toEqual({
             $os: 'Windows',
             $os_name: 'Android',
             $set_once: { $initial_os: 'Windows' },
             $set: { $os: 'Windows' },
         })
     })
+    it('falls back to $os_name for $initial_os when the event carries a null $os', () => {
+        // The never-write-null rule skips $initial_os for a null $os, leaving $os_name as the only
+        // first-touch value available.
+        const properties = { $os: null, $os_name: 'Android' }
+        expect(personInitialAndUTMProperties(properties)).toEqual({
+            $os: null,
+            $os_name: 'Android',
+            $set_once: { $initial_os: 'Android' },
+            $set: { $os: null },
+        })
+    })
     it('does not lift device context from an $is_server event, but keeps campaign params', () => {
         // A server SDK (is_server: true) stamps its host $os; it must not reach $set/$set_once or
         // it permanently poisons $initial_os. utm is real attribution and still flows through.
@@ -156,7 +161,7 @@ describe('personInitialAndUTMProperties()', () => {
             { gclid: null, msclkid: null },
             {},
         ],
-        ['a null $os_name does not claim $initial_os', { $os_name: null }, { $os: null }, {}],
+        ['a null $os_name does not claim $initial_os', { $os: null, $os_name: null }, { $os: null }, {}],
     ])('never writes null first-touch values: %s', (_desc, input, expectedSet, expectedSetOnce) => {
         const result = personInitialAndUTMProperties({ ...input })
         expect(result.$set).toEqual(expectedSet)
@@ -165,7 +170,6 @@ describe('personInitialAndUTMProperties()', () => {
 
     it.each([
         ['client event maps $os', { $lib: 'web', $os: 'Linux' }, 'Linux'],
-        ['client mobile via $os_name', { $lib: 'posthog-ios', $os_name: 'iOS' }, 'iOS'],
         ['no $is_server is treated as client', { $os: 'Linux' }, 'Linux'],
         ['$is_server false is treated as client', { $os: 'Linux', $is_server: false }, 'Linux'],
         ['$is_server true skips $os', { $os: 'Linux', $is_server: true }, undefined],
diff --git a/rust/feature-flags/src/flags/flag_matching_utils.rs b/rust/feature-flags/src/flags/flag_matching_utils.rs
index 92dbef67e242..acf03d365296 100644
--- a/rust/feature-flags/src/flags/flag_matching_utils.rs
+++ b/rust/feature-flags/src/flags/flag_matching_utils.rs
@@ -166,8 +166,10 @@ pub fn populate_missing_initial_properties(properties: &mut HashMap<String, Valu
 ///
 /// Web SDKs (posthog-js) report the OS as `$os`; mobile SDKs report it as
 /// `$os_name`. Ingestion normalizes `$os_name` -> `$os` at write time
-/// (`personInitialAndUTMProperties` in nodejs/src/utils/db/utils.ts), but that
-/// only covers newly-written rows — historical and un-normalized person rows can
+/// (`normalizeOsAlias` in nodejs/src/common/utils/event.ts for the event's own
+/// `$os`, `personInitialAndUTMProperties` in nodejs/src/common/utils/db/utils.ts
+/// for the person row), but that only covers newly-written rows, and it excludes
+/// `$is_server` events; historical and un-normalized person rows can
 /// carry only one of the two keys. Flag matching does exact-key lookups, so
 /// without this a mobile person whose row has only `$os_name` never matches an
 /// `$os` filter (and vice versa). The mirror is bidirectional so either filter