diff --git a/Library/Homebrew/test/extend/on_system_spec.rb b/Library/Homebrew/test/extend/on_system_spec.rb
new file mode 100644
index 0000000000..605c37f0d8
--- /dev/null
+++ b/Library/Homebrew/test/extend/on_system_spec.rb
@@ -0,0 +1,176 @@
+# frozen_string_literal: true
+
+require "extend/on_system"
+
+RSpec.describe OnSystem do
+  shared_examples "a class with on_arch methods" do
+    it "adds on_arch methods to class for `ARCH_OPTIONS`" do
+      OnSystem::ARCH_OPTIONS.each do |arch|
+        expect(subject.method_defined?(:"on_#{arch}")).to be true
+      end
+    end
+  end
+
+  shared_examples "a class with on_base_os methods" do
+    it "adds on_os methods to class for `BASE_OS_OPTIONS`" do
+      OnSystem::BASE_OS_OPTIONS.each do |os|
+        expect(subject.method_defined?(:"on_#{os}")).to be true
+      end
+    end
+  end
+
+  shared_examples "a class with on_macos methods" do
+    it "adds on_os methods to class for `MacOSVersion::SYMBOLS` keys" do
+      MacOSVersion::SYMBOLS.each_key do |os|
+        expect(subject.method_defined?(:"on_#{os}")).to be true
+      end
+    end
+  end
+
+  describe "::arch_condition_met?" do
+    it "returns true if current arch equals provided arch" do
+      Homebrew::SimulateSystem.with(arch: :arm) do
+        expect(described_class.arch_condition_met?(:arm)).to be true
+      end
+    end
+
+    it "returns false if current arch does not equal provided arch" do
+      Homebrew::SimulateSystem.with(arch: :arm) do
+        expect(described_class.arch_condition_met?(:intel)).to be false
+      end
+    end
+
+    it "raises error if provided arch is not valid" do
+      expect { described_class.arch_condition_met?(:nonexistent_arch) }
+        .to raise_error(ArgumentError)
+    end
+  end
+
+  describe "::os_condition_met?" do
+    let(:newest_macos) { MacOSVersion::SYMBOLS.keys.first }
+
+    it "returns result of `SimulateSystem.simulating_or_running_on_<os_name>` for supported OS" do
+      Homebrew::SimulateSystem.with(os: newest_macos) do
+        # This needs to use a value from `BASE_OS_OPTIONS`
+        expect(described_class.os_condition_met?(:macos)).to be true
+      end
+    end
+
+    it "returns false if `os_name` is a macOS version but OS is Linux" do
+      Homebrew::SimulateSystem.with(os: :linux) do
+        expect(described_class.os_condition_met?(newest_macos)).to be false
+      end
+    end
+
+    it "returns false if current OS is `:macos` and `os_name` is a macOS version" do
+      # A generic macOS version is treated as less than any other version.
+      Homebrew::SimulateSystem.with(os: :macos) do
+        expect(described_class.os_condition_met?(newest_macos)).to be false
+      end
+    end
+
+    it "returns true if current OS equals the `os_name` macOS version" do
+      Homebrew::SimulateSystem.with(os: newest_macos) do
+        expect(described_class.os_condition_met?(newest_macos)).to be true
+      end
+    end
+
+    it "returns true if current OS meets the `or_condition` for `os_name` macOS version" do
+      current_os = :ventura
+      Homebrew::SimulateSystem.with(os: current_os) do
+        expect(described_class.os_condition_met?(current_os, :or_newer)).to be true
+        expect(described_class.os_condition_met?(:big_sur, :or_newer)).to be true
+        expect(described_class.os_condition_met?(current_os, :or_older)).to be true
+        expect(described_class.os_condition_met?(newest_macos, :or_older)).to be true
+      end
+    end
+
+    it "returns false if current OS does not meet the `or_condition` for `os_name` macOS version" do
+      Homebrew::SimulateSystem.with(os: :ventura) do
+        expect(described_class.os_condition_met?(newest_macos, :or_newer)).to be false
+        expect(described_class.os_condition_met?(:big_sur, :or_older)).to be false
+      end
+    end
+
+    it "raises an error if `os_name` is not valid" do
+      expect { described_class.os_condition_met?(:nonexistent_os) }
+        .to raise_error(ArgumentError, /Invalid OS condition:/)
+    end
+
+    it "raises an error if `os_name` is a macOS version but `or_condition` is not valid" do
+      expect do
+        described_class.os_condition_met?(newest_macos, :nonexistent_condition)
+      end.to raise_error(ArgumentError, /Invalid OS `or_\*` condition:/)
+    end
+  end
+
+  describe "::condition_from_method_name" do
+    it "returns a symbol with any `on_` prefix removed" do
+      expect(described_class.condition_from_method_name(:on_arm)).to eq(:arm)
+    end
+
+    it "returns provided symbol if there is no `on_` prefix" do
+      expect(described_class.condition_from_method_name(:arm)).to eq(:arm)
+    end
+  end
+
+  describe "::setup_arch_methods" do
+    subject do
+      klass = Class.new
+      described_class.setup_arch_methods(klass)
+      klass
+    end
+
+    it_behaves_like "a class with on_arch methods"
+  end
+
+  describe "::setup_base_os_methods" do
+    subject do
+      klass = Class.new
+      described_class.setup_base_os_methods(klass)
+      klass
+    end
+
+    it_behaves_like "a class with on_base_os methods"
+  end
+
+  describe "::setup_macos_methods" do
+    subject do
+      klass = Class.new
+      described_class.setup_macos_methods(klass)
+      klass
+    end
+
+    it_behaves_like "a class with on_macos methods"
+  end
+
+  describe "::included" do
+    it "raises an error" do
+      expect { Class.new { include OnSystem } }
+        .to raise_error(/Do not include `OnSystem` directly/)
+    end
+  end
+
+  describe "::MacOSAndLinux" do
+    subject { Class.new { include OnSystem::MacOSAndLinux } }
+
+    it "can be included" do
+      expect { Class.new { include OnSystem::MacOSAndLinux } }.not_to raise_error
+    end
+
+    it_behaves_like "a class with on_arch methods"
+    it_behaves_like "a class with on_base_os methods"
+    it_behaves_like "a class with on_macos methods"
+  end
+
+  describe "::MacOSOnly" do
+    subject { Class.new { include OnSystem::MacOSAndLinux } }
+
+    it "can be included" do
+      expect { Class.new { include OnSystem::MacOSOnly } }.not_to raise_error
+    end
+
+    it_behaves_like "a class with on_arch methods"
+    it_behaves_like "a class with on_macos methods"
+  end
+end