If I have Onslaught in play and I cast Centaur Omenreader can that be the creature I tap?
If I have Onslaught in play and I cast Centaur Omenreader can that be the creature I tap?
No. Onslaught triggers when you cast the spell, meaning the Omenreader has not yet resolved and isn't on the battlefield when you choose the target.
Fluttershy is best pony.