Kaitlin Olson is going from FXX to FOX.

The It’s Always Sunny in Philadelphia star will recur on New Girl Season 4... as Jess' worst nightmare!

Look for Olson to appear as Ashley, Jess and Cece’s former high school nemesis who is now engaged to Jess’ father Bob (returning guest star Rob Reiner). Awkward!

Olson joins previously announced New Girl Season 4 guest stars Jessica Biel and Reid Scott (who will appear on the September 16 opener) and she will debut in Episode 3.

Need to catch up before then? You can always watch New Girl online.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.