I'm sorry if this has been covered before, but when I used cookies in a buttercream filling, will they end up mushy/soggy? TIA.

Actually I've not done it but my guess would be yes. It might depend a bit on the cookie used.......some might get mushy more than others.
Why not do a test yourself? Just take a couple spoonsfull of icing, mix in one cookie and let it sit overnight (or similar time frame) to see what happens.

LOL, why on earth didn't I think of that?
That's kind of what I figured, but I'll still give it a try to see if it's Okay-mushy or gross-mushy. I'm making two cakes thursday for friday, one w/nilla wafer filling and one w/oreo.

Thanks for the response/idea.