Mr. Satan is building a hotel in honor of his defeating Buu, and the press seem to know of Buu's existence. But Shenron erased everyone's memories of Buu following his defeat.
Elder Kai, King Kai, Bubbles, and Gregory can all be seen at the party. But they are dead and have no way of coming to Earth (there's even a set of dialogue in the Buu Saga about how Elder Kai is giving Goku his life because there is no way Elder Kai could return to Earth anyway.)
Gotenks is able to de-fuse instantly after the failed "fat" fusion. But in the series when fighting Buu, and in the movie with Janemba, fusing, successful or not, lasts half an hour. There's no way to end a fusion early, nor is there a way to extend the effects.
Lunch appears the same as she does in Dragon Ball, however in the end of Dragon Ball Z, she appears older, despite the OVA taking place after the Buu Saga.