diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 6a69e81..c643711 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -16,6 +16,7 @@
 │   ├── AssemblyInfo.fs             – assembly metadata
 │   ├── ProcessingOptions.fs        – processing configuration helpers
 │   ├── Observable.fs               – observable wrappers/extensions
+│   ├── ObservableFactories.fs      – factories reachable as `Observable.xxx` (`ofSeq`)
 │   ├── AsyncObservable.fs          – async observable helpers
 │   └── TaskObservable.fs           – task-based observable helpers
 ├── tests/FSharp.Control.R3.Tests/  – MSTest test project
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 63d3ec8..9d4f319 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,11 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `Observable.ofSeq (items)` and `Observable.ofSeq (items, cancellationToken)`, reachable after `open FSharp.Control.R3`
 - `rxqueryWith cancellationToken` and `RxQueryBuilder (cancellationToken)` to cancel the query operators that return a task
 
 ### Changed
 
 - **Breaking:** `AwaitOperationConfiguration` cases are prefixed with `Await` (`AwaitSequential`, `AwaitParallel 4`, ...) so they no longer collide with `System.Threading.Tasks.Parallel`
+- **Breaking:** the `Observable.Extensions` type is replaced by `Observable.ofSeq`
+- **Breaking:** after `open FSharp.Control.R3`, the bare type name `Observable` refers to `FSharp.Control.R3.ObservableFactories.Observable`; write `open type R3.Observable` and extend `R3.Observable` by its full name
 - **Breaking:** `rxquery` `sumBy` adds values of the projected type: it requires `(+) : 'Value * 'Value -> 'Value`
 - `mapAsync` and `iterAsync` validate the concurrency limit of the options when they are called
 
diff --git a/src/FSharp.Control.R3/FSharp.Control.R3.fsproj b/src/FSharp.Control.R3/FSharp.Control.R3.fsproj
index c8c433d..cb270e1 100644
--- a/src/FSharp.Control.R3/FSharp.Control.R3.fsproj
+++ b/src/FSharp.Control.R3/FSharp.Control.R3.fsproj
@@ -19,6 +19,7 @@
     <Compile Include="AssemblyInfo.fs" />
     <Compile Include="ProcessingOptions.fs" />
     <Compile Include="Observable.fs" />
+    <Compile Include="ObservableFactories.fs" />
     <Compile Include="AsyncObservable.fs" />
     <Compile Include="TaskObservable.fs" />
   </ItemGroup>
diff --git a/src/FSharp.Control.R3/Observable.fs b/src/FSharp.Control.R3/Observable.fs
index 003921f..3078610 100644
--- a/src/FSharp.Control.R3/Observable.fs
+++ b/src/FSharp.Control.R3/Observable.fs
@@ -75,15 +75,6 @@ let inline take (count : int) (source) = ObservableExtensions.Take (source, coun
 let inline where ([<InlineIfLambda>] f : 'T -> bool) source = ObservableExtensions.Where (source, f)
 
 open System.Runtime.CompilerServices
-open System.Runtime.InteropServices
-
-[<AutoOpen>]
-module Extensions =
-
-    [<AbstractClass; Sealed; Extension>]
-    type Observable private () =
-
-        static member ofSeq (items : _ seq, [<Optional>] cancellationToken) = Observable.ToObservable (items, cancellationToken)
 
 [<AutoOpen>]
 module OptionExtensions =
diff --git a/src/FSharp.Control.R3/ObservableFactories.fs b/src/FSharp.Control.R3/ObservableFactories.fs
new file mode 100644
index 0000000..97c9108
--- /dev/null
+++ b/src/FSharp.Control.R3/ObservableFactories.fs
@@ -0,0 +1,46 @@
+namespace FSharp.Control.R3
+
+open System.Threading
+open R3
+
+/// <summary>
+/// Factories of observable sequences that are reachable through the type name <c>Observable</c>
+/// as soon as <see cref="N:FSharp.Control.R3"/> is opened.
+/// <para>
+/// This module sits directly in the namespace, so opening the namespace opens it too.
+/// Name resolution of <see cref="M:FSharp.Control.R3.ObservableFactories.Observable.ofSeq``1(System.Collections.Generic.IEnumerable{``0})"/>
+/// finds no such function in the modules named <c>Observable</c> and falls through to the type
+/// <see cref="T:FSharp.Control.R3.ObservableFactories.Observable"/>, the same way
+/// <see cref="M:FSharp.Control.R3.Task.Extensions.Observable.ofTask``1(Microsoft.FSharp.Core.FSharpFunc{System.Threading.CancellationToken,System.Threading.Tasks.ValueTask{``0}},System.Boolean)"/>
+/// resolves after opening <see cref="T:FSharp.Control.R3.Task"/>. R3's own factories, such as
+/// <see cref="M:R3.Observable.Range(System.Int32,System.Int32)"/>, still resolve through the same name.
+/// </para>
+/// <para>
+/// Where the bare type name matters, it now refers to this type: write <c>open type R3.Observable</c> rather than
+/// <c>open type Observable</c>, and extend <see cref="T:R3.Observable"/> by its full name.
+/// </para>
+/// </summary>
+[<AutoOpen>]
+module ObservableFactories =
+
+    /// Factories of observable sequences.
+    [<AbstractClass; Sealed>]
+    type Observable private () =
+
+        /// <summary>
+        /// Creates an observable sequence that emits the elements of <paramref name="items"/> synchronously on every subscription
+        /// and then completes.
+        /// <para>The sequence is enumerated to the end even when the subscriber unsubscribes earlier, so it must be finite.</para>
+        /// </summary>
+        static member ofSeq (items : 'T seq) : Observable<'T> = R3.Observable.ToObservable items
+
+        /// <summary>
+        /// Creates an observable sequence that emits the elements of <paramref name="items"/> synchronously on every subscription
+        /// and then completes.
+        /// <para>
+        /// Once <paramref name="cancellationToken"/> is cancelled, no further element is emitted and the sequence completes successfully.
+        /// The token is checked after the enumerator moved to the next element, so the side effects of producing that element still run.
+        /// </para>
+        /// </summary>
+        static member ofSeq (items : 'T seq, cancellationToken : CancellationToken) : Observable<'T> =
+            R3.Observable.ToObservable (items, cancellationToken)