$ brew install babashka/brew/obb
Running `brew update --auto-update`...

==> Auto-updated Homebrew!
Updated 1 tap (homebrew/core).

You have 60 outdated formulae and 7 outdated casks installed.
You can upgrade them with brew upgrade
or list them with brew outdated.

==> Downloading 
Already downloaded: /Users/rkirchofer/Library/Caches/Homebrew/downloads/32ab0db39c2cffc5046614247fa805bbba9947f9ed68b8b109fd309dfeed9aad--obb.tar.gz
==> Installing obb from babashka/brew
Error: Your Xcode (12.5.1) is too outdated.
Please update to Xcode 13.4 (or delete it).
Xcode can be updated from the App Store.e

That message is coming from brew, of course. I'm not sure it's possible to circumvent that restriction.

Oh, so it not even to do with obb. Never mind then. Thanks!