Why not return to the team O'Neill was drafted by? Seattle made big noise last season, ending their playoff drought and proving to be one of the best up-and-coming teams in all of baseball.
While Julio Rodriguez is the face of their franchise, it's their starting pitching that has the best depth in my opinion. Luis Castillo, Logan Gilbert, George Kirby, Robbie Ray, and Marco Gonzales make up an elite group of starters that can carry the team, and they even have guys like Chris Flexen who can slot on in for them.
Sure, injuries happen, but right now they have an abundance of starters, and could use some more thump in their lineup. While Marco Gonzales originally was sent to Seattle to acquire O'Neill, I doubt the Cardinals' would give their outfielder back to the Mariners without getting one of Castillo, Kirby, Gilbert, or Ray in return.
Would Seattle consider this? I'm not sure. I think it would be hard for the Mariners to find another buyer who is willing to give up big bats like O'Neill. Unless he goes on a tear in the next few months and proves he's back to his 2021 self, it would likely require more talent coming from St. Louis as well, which they could be more than happy to swap.