OpenDisplay is an open-source software tool that lets users turn their iPhone, iPad, or older Mac into a secondary display for their main Mac. This is similar to Apple's Sidecar feature, which allows an iPad to act as a screen for a Mac, but OpenDisplay has some key differences. It supports multiple Apple accounts and can use older devices, unlike Sidecar, which requires both devices to be on the same Apple account and only works with iPads. OpenDisplay is free and available under the GPL license, which allows users to freely use, modify, and distribute the software. To use OpenDisplay, a companion app called OpenDisplay Receiver must be installed on the secondary device, which turns it into a screen. The secondary device needs to run macOS 12 Monterey or later, while the Mac sending the display must be on macOS 14 or later. For iOS devices, the app is available on the App Store for iOS 16.4 and later. The connection between the devices uses usbmuxd, a built-in macOS service that handles communication between devices, so no third-party tools or tunnels are needed. However, a data transfer cable is required—only charging-only cables will not work. Once connected, the secondary device becomes touch-sensitive, allowing users to perform gestures like scrolling with two fingers. While OpenDisplay can work over Wi-Fi, there is a known issue with the mouse cursor stuttering. Additionally, the video stream over Wi-Fi is unencrypted, which could pose a security risk on shared or public networks. The developers plan to implement an encrypted connection with device pairing, but this feature is not yet available. The Mac application uses CGVirtualDisplay, a private API from Apple's CoreGraphics framework. This same API is used by DeskPad, a similar tool that also isn't available on the App Store. Because it relies on a private API, OpenDisplay cannot be listed on the App Store, and it could potentially stop working if Apple updates macOS and changes or removes the API.