I do Opah lazy style, but the wife loves it this way:
Tablespoon of butter or a bit more, 'bout same olive oil. Heat up to around medium (guess) or a bit less and I toss in some diced garlic. When lazy (usually) just use dried diced garlic, and let it steep in the butter a while. Do not go too hot, been there, regretted it, overcooks the fish surface before the inside is done.
Season the fish first both sides with garlic salt (yeah, we like garlic) and pepper first. Added smoked paprika a few times for variety.
Ease it into the pan. If less than an inch thick, about three minutes on each side. You can see the fish cook.
After about two minutes on the the second side, I kill the heat, toss in a tablespoon or so of white wine and cover it for another couple minutes and let it steam while the wife farts around with the rest of the dinner.
Works well most all the time. I know my stove settings, so that is up to you and your equipment. Thick stuff like what I just did tonight from Costco that was about 1.5" a went up to five minutes per side and was told it was perfect.
Opah does not have a lot of inherent flavor like Ono so it can be killed with too much spice. This was trial and error for me, so YMMV. The wife's killer tarter sauce works perfectly with this simple saute.