Well, I don't. I just found out today that the last ep of that show apparently indicates that the whole series was the (day?)dream of a child.
Now, if you say something happened during a dream, that means it's not real, right...?
Okay -- then Oz, all the Law & Order series, Seinfeld, Picket Fences, etc., they all never happened.
Follow the logic here, but it's fairly reasonable. If character A showed up on Elsewhere, and (we'll say) showed up on Show B, then B never existed. If B never existed, and character C on show B appears on another show, then that show never existed either, and vice versa -- if someone from Show Z appeared on Elsewhere, they never existed, so their show never existed, so...
Moral of the story: Be careful who you do TV crossovers with -- you're crossing over with all their crossovers. Practice safe role reprisal!