diff --git a/app_to_web.md b/app_to_web.md
index 1f7e8210cc..19f072dc95 100644
--- a/app_to_web.md
+++ b/app_to_web.md
@@ -54,17 +54,16 @@ background requests will be made and the browser will not set
 `Attribution-Reporting-Eligible` header on `<a>`, `window.open`, `<img>`, or
 `<script>` requests.
 
-If the `Attribution-Reporting-Support` header indicates OS support, the reporting origin can optionally respond to the request with a [string structured header](https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-header-structure-15#section-3.3.3) that indicates a desire to use the OS's attribution API instead of the browser's. Note that the API also allows browsers to only support OS-level attribution if they choose.
-```
+If the `Attribution-Reporting-Support` header indicates OS support, the reporting origin can optionally respond to the request with a [list structured header](https://httpwg.org/specs/rfc8941.html#list) containing one or more URLs that indicates a desire to use the OS's attribution API instead of the browser's. Note that the API also allows browsers to only support OS-level attribution if they choose.
+```http
 // Registers a source against a native OS attribution API
-Attribution-Reporting-Register-OS-Source: "https://adtech.example/register-android-source?..."
-
+Attribution-Reporting-Register-OS-Source: "https://adtech.example/register", "https://other-adtech.example/register"
 ```
 
 Trigger registrations will accept a new response header as well:
-```
+```http
 // Registers a trigger against a native OS attribution API
-Attribution-Reporting-Register-OS-Trigger: "https://adtech.example/register-android-trigger?..."
+Attribution-Reporting-Register-OS-Trigger: "https://adtech.example/register", "https://other-adtech.example/register"
 ```
 
 After receiving these headers, the browser will pass these URLs into the underlying OS API with any additional information including:
diff --git a/header-validator/test.os.js b/header-validator/test.os.js
index 816d2ed291..8bee21382e 100644
--- a/header-validator/test.os.js
+++ b/header-validator/test.os.js
@@ -6,6 +6,7 @@ const tests = [
   {input: '"https://a.test/"'},
   {input: '"http://localhost/"'},
   {input: '"http://127.0.0.1/"'},
+  {input: '"https://a.test/", "https://b.test/"'},
 
   // Warnings
   {
@@ -30,6 +31,13 @@ const tests = [
       msg: 'must be a string',
     }],
   },
+  {
+    input: '("https://a.test/")',
+    errors: [{
+      path: [],
+      msg: 'must be a string',
+    }],
+  },
 
   // Invalid URL
   {
diff --git a/header-validator/validate-os.js b/header-validator/validate-os.js
index 08ef766509..cbd519193e 100644
--- a/header-validator/validate-os.js
+++ b/header-validator/validate-os.js
@@ -1,17 +1,17 @@
-async function getParseItemFunctionFromEnvironment() {
-  let parseItem = null
+async function getParseListFunctionFromEnvironment() {
+  let parseList = null
   if (typeof window === 'undefined') {
     // Environment = nodeJS -> Take structured header functions from the locally installed node module
     const structuredHeaderLib = await import('structured-headers')
-    parseItem = structuredHeaderLib.default.parseItem
+    parseList = structuredHeaderLib.default.parseList
   } else {
     // Environment = browser -> Take structured header functions from the lib loaded from the CDN (see HTML)
-    parseItem = window.structuredHeader.parseItem
+    parseList = window.structuredHeader.parseList
   }
-  return parseItem
+  return parseList
 }
 
-const parseItem = await getParseItemFunctionFromEnvironment()
+const parseList = await getParseListFunctionFromEnvironment()
 
 function validateString(item) {
   if (typeof item !== 'string') {
@@ -46,29 +46,31 @@ function validate(str, paramChecks) {
   const errors = []
   const warnings = []
 
-  let item, params
+  let list
   try {
-    [item, params] = parseItem(str)
+    list = parseList(str)
   } catch (err) {
     errors.push({ msg: err.toString() })
     return { errors, warnings }
   }
 
-  const err = validateURL(item)
-  if (err) {
-    errors.push({ msg: err, path: [] })
-  }
-
-  Object.entries(paramChecks).forEach(([param, check]) => {
-    const err = check(params.get(param))
+  for (const [item, params] of list) {
+    const err = validateURL(item)
     if (err) {
-      errors.push({ msg: err, path: [param] })
+      errors.push({ msg: err, path: [] })
     }
-    params.delete(param);
-  });
 
-  for (const key of params.keys()) {
-    warnings.push({ msg: 'unknown parameter', path: [key] })
+    Object.entries(paramChecks).forEach(([param, check]) => {
+      const err = check(params.get(param))
+      if (err) {
+        errors.push({ msg: err, path: [param] })
+      }
+      params.delete(param);
+    });
+
+    for (const key of params.keys()) {
+      warnings.push({ msg: 'unknown parameter', path: [key] })
+    }
   }
 
   return { errors, warnings }
diff --git a/index.bs b/index.bs
index 447a99439b..e40fa21ef1 100644
--- a/index.bs
+++ b/index.bs
@@ -1221,19 +1221,19 @@ To <dfn>process an attributionsrc response</dfn> given a [=suitable origin=]
 1. Let |triggerHeader| be the result of [=header list/get|getting=]
     "`Attribution-Reporting-Register-Trigger`" from |response|'s
     [=response/header list=].
-1. Let |osSourceURL| be the result of
-    [=get an OS-registration URL from a header list|getting an OS-registration URL=]
+1. Let |osSourceURLs| be the result of
+    [=get OS-registration URLs from a header list|getting OS-registration URLs=]
     from |response|'s [=response/header list=] with
     "`Attribution-Reporting-Register-OS-Source`".
-1. Let |osTriggerURL| be the result of
-    [=get an OS-registration URL from a header list|getting an OS-registration URL=]
+1. Let |osTriggerURLs| be the result of
+    [=get OS-registration URLs from a header list|getting OS-registration URLs=]
     from |response|'s [=response/header list=] with
     "`Attribution-Reporting-Register-OS-Trigger`".
 1. If |eligibility| is:
     <dl class="switch">
     : "<code>[=eligibility/navigation-source=]</code>"
     :: Run the following steps:
-        1. If |sourceHeader| and |osSourceURL| are both null or both not
+        1. If |sourceHeader| and |osSourceURLs| are both null or both not
             null, return.
         1. If |sourceHeader| is not null:
             1. Let |source| be the result of running
@@ -1243,13 +1243,13 @@ To <dfn>process an attributionsrc response</dfn> given a [=suitable origin=]
                 [=current wall time=].
             1. If |source| is not null, [=process an attribution source|process=]
                 |source|.
-        1. If |osSourceURL| is not null and the user agent supports OS
-            registrations, process |osSourceURL| according to an
+        1. If |osSourceURLs| is not null and the user agent supports OS
+            registrations, process |osSourceURLs| according to an
             [=implementation-defined=] algorithm.
     : "<code>[=eligibility/event-source-or-trigger=]</code>"
     :: Run the following steps:
         1. If the number of non-null entries in «|sourceHeader|,
-            |triggerHeader|, |osSourceURL|, |osTriggerURL|» is not 1, return.
+            |triggerHeader|, |osSourceURLs|, |osTriggerURLs|» is not 1, return.
         1. If |sourceHeader| is not null:
             1. Let |source| be the result of running
                 [=parse source-registration JSON=] with |sourceHeader|,
@@ -1267,11 +1267,11 @@ To <dfn>process an attributionsrc response</dfn> given a [=suitable origin=]
                 |destinationSite|, |reportingOrigin|, |privateStateTokens|, and
                 |context|'s [=current wall time=].
             1. If |trigger| is not null, [=trigger attribution=] with |trigger|.
-        1. If |osSourceURL| is not null and the user agent supports OS
-            registrations, process |osSourceURL| according to an
+        1. If |osSourceURLs| is not null and the user agent supports OS
+            registrations, process |osSourceURLs| according to an
             [=implementation-defined=] algorithm.
-        1. If |osTriggerURL| is not null and the user agent supports OS
-            registrations, process |osTriggerURL| according to an
+        1. If |osTriggerURLs| is not null and the user agent supports OS
+            registrations, process |osTriggerURLs| according to an
             [=implementation-defined=] algorithm.
 
     </dl>
@@ -2967,14 +2967,20 @@ A user agent MAY retry this algorithm in the event that there was an error.
 
 # Cross App and Web Algorithms # {#cross-app-and-web}
 
-To <dfn noexport>get an OS-registration URL from a header list</dfn> given a
+To <dfn noexport>get OS-registration URLs from a header list</dfn> given a
 [=header list=] |headers| and a [=header name=] |name|:
 
-1. Let |value| be the result of
+1. Let |values| be the result of
     [=header list/get a structured field value|getting=] |name| from |headers|
-    with a type of "`item`".
-1. If |value| is not a [=string=], return null.
-1. Return the result of running the [=URL parser=] on |value|.
+    with a type of "`list`".
+1. If |values| is not a [=list=], return null.
+1. Let |urls| be a new [=list=].
+1. [=list/iterate|For each=] |value| of |values|:
+     1. If |value| is not a [=string=], [=iteration/continue=].
+     1. Let |url| be the result of running the [=URL parser=] on |value|.
+     1. If |url| is failure or null, [=iteration/continue=].
+     1. [=list/Append=] |url| to |urls|.
+1. Return |urls|.
 
 To <dfn>get supported registrars</dfn>: