Convert OOF.mp3 to an .ogg file. Change the file name to ouch.ogg, then drag it into your Roblox Folder and replace the Ouch.ogg file. Start up Roblox to see if the Oof sound has been added back in.

Evidently, though, Tallarico and the Roblox Corporation have worked something new out, as the "oof" sound is now once again available for use. The specifics of whatever deal was made aren't clear ...

The Roblox "oof" sound, which became famous not just with players but around the internet as a meme, has been removed from the game entirely due to a licensing issue.