I haven't seen many Open Bar calves but from what I can see he needs to be used on a cow with good rib shape. In my opinion the Maverick calves are inconsistent. The good ones are good, but there are 2 below average calves for every good one. He seems to work best on cows that have plenty of power and performance.
Tyson, K&A Nautica, and TLM Bouncer are all great recommendations. Bouncer is very easy calving so use him if that is a concern. Be sure to look into Tyson sons too, there are starting to be a few of them around.
Also, I would look at Rowe Gold Club. He is a PHA carrier but it might be worth it to get the cow tested and if she is free use him.