ou relays on no one they are a legit organization best running kashrus agency in the usa maybe even the world
The OU definitely has some very good things about their standards, procedures, and transparency, but they are far from perfect.
I'm no expert in the world of Kashrus, but based on my understanding there are different areas, each requiring a somewhat different expertise and skill-set.
1. Food manufacturing plants. This is where the OU excel (and I believe the OK does very well too) with their extensive knowledge and capabilities in modern food technology manufacturing, wide reach and extensive database. And in these cases you are somewhat correct in that when there's an OU and another supervision, the other supervision relies heavily on the OU, sometimes they add certain extra stringencies (such as rejecting certain runs), but overall for the ingredients and maintaining overall kosher production, they rely on the OU.
2. Local food establishment supervision. This is a totally different story. The OU does have the standards and procedures, but in these cases it is much more dependent on the local Mashgiach and the owner of the establishment. I don't know what their standards and procedures are, but I'm sure they are quite transparent and can be asked. In this area they are not necessarily better or worse than others, they just have their standards. It's not common to have a dual supervision on a food establishment, but it's likely that in places where those exist, the OU might be relying on the other supervision for mashgiach hiring and oversight.
3. Shechita. This is a totally different ballgame. As hinted in another thread I started, shechita cannot objectively be deemed good or not good, it's highly subjective as it relies on the Yiras Shomaim of the Shochet (the bedika and kashering could definitely be deemed objective, but there's no-way one can call Shochet screening objective. Yes, there is an objective part to it in preparing the Shechita knife, but the actual מעשה השחיטה depends on the Yiras Shomaim of the Shoechet.) It would be an extremely long stretch to think that anyone that really cares about Glatt Kosher meat would rely on the OU for that, especially if talking about an operation that's not the size of Empire Chicken.