Since Mike barely knew Oscar it would indeed be possible to forget the guy. Archie himself didn't know much about Oscar and he was his cousin. I have plenty of cousins that I've met but wouldn't know them if I tripped over them.
Mike asks Archie who's Cousin Oscar, who is the cousin that died under the 704 roof back in Season 2. The goof is, Mike and Gloria were under the roof back then and they even attended the funeral which Archie hosted--how can Mike not remember?
This episode was videotaped on February 25, 1977.
Sally Struthers does not appear in this episode.