Activate Xavier Sal's second ability by tapping it and sacrificing any other creature you control.
When the creature dies, Xavier Sal triggers, untapping Xavier Sal.
Resolve the Xavier Sal ability from step 1, putting a +1/+1 counter on Scurry Oak.
Scurry Oak triggers, creating a 1/1 Squirrel creature token.
Repeat.
Once you have infinite +1/+1 counters on Scurry Oak, you may instead use Xavier Sal's first ability to get infinite copies of creature tokens you control.