Oz and Spike cross over to AtS in In the Dark, episode 3 of s1.
Angel crosses over to Buffy during Pangs, BtVS s4 episode 8, but there's no direct interaction between the two. Just him and the Scoobies.
Then, right after that -
Buffy crosses over to Angel in I Will Remember You, which is like AtS s1, episode 8. If you ship B/A, this will be a good episode for ya'll.
Then no crossovers until after the events of Who Are You, BtVS s4, episode 16. Faith leaves Sunnydale and runs to LA, where -
She encounters Angel in 5x5.
Buffy crosses over in the next AtS episode Sanctuary, AtS s1, episode 19.
Then, right after that, Angel crosses over in The Yoko Factor, BtVS s4, episode20, for a couple of scenes.
This concludes the crossover extravaganza for those two seasons.
If you ship B/A, they are awesome appearances. However, I always felt there was too many crossovers between the two shows during that first year for AtS. AtS was a different show, with it's own mission statement, and it needed to stand on its own two feet without appearances on the parent show, or guest stars from the parent show. /.02 cents.
After this, there's -
In BtVS s5, Angel (during AtS's s2) crosses over in Forever - a wonderful appearance that made sense, and that I loved.
In AtS s4 (this is the season that triggers my gag reflex, so excuse any poison that may spew from my guts), Faith appears again, then Willow crosses over - to help out with the Big Bad and to get Faith and take her back to Sunnydale. You don't need to watch BtVS this season (s7) to understand Willow's appearance. It's insular to AtS. I don't remember the episode title or numbers, mostly because I tried hard to forget that whole, stupid arc (imo), so I can't help there.
One last crossover by Angel to BtVS happens in the Buffy finale, Chosen. Only the teaser, though. He doesn't stick around. This has nothing to do with events in AtS - much. Just one gawdy piece of jewlrey.
Again, the only crossovers that you may have to watch both shows to fully understand happens during Angel's first season, running concurrent with Buffy's fourth season. But there's enough exposition on the episodes in question for you to understand the context.
Hope this helps.











