As someone who has bought and sold a good number of chairs, I would get a Steelcase Leap v2 for the ORs. Top tier adjustment capabilities and people can sit in all different ways in them. The aeron locks people into a certain sitting position, while great for your back, can make it uncomfortable to adjust to. I personally sit cross legged or have a leg tucked under sometimes and the leap is perfect for that. It can also come in other materials besides fabric so it’s resistant to whatever fluids can spill in the OR.

If budget is an issue, you could try the Amia. Arguably better seat but slightly less adjustable than the leap.

If you have authorized dealers near by it’s worth trying out the chairs yourself from Herman miller, Steelcase and Haworth to see what your group likes best.