You need the original PC or Mac retail installation files. Since Oni is currently "abandonware" and not available on digital storefronts like Steam or GOG, you will need your original game disc or an ISO rip of the retail game.
The AE fixes numerous bugs left by the original developers and provides support for modern Windows and Intel-based Mac systems.
To successfully play the Anniversary Edition, you must follow a specific sequence: obtain the original game, download the AE installer, and deploy the patches. Step 1: Prepare the Base Game