Criticizing every move she made, belittling her career (especially if he were the military spouse who should've know what he was getting into when he got married, and decided if he could deal with that for better or for worse, or not), and constantly making "jokes" about how she'd better not cheat on him, as if automatically assuming the intention to cheat is there. I would find O'Brien unwatchable if the exact same behaviors were taken and then scripted onto him, if all he ever did was try to break her down.
The only time O'Brien was an real a-hole to Keiko was in
Fascination, but even O'Brien realised he crossed the line as show in this script taken from
Fascination:
Look, you're right. I'm an idiot, sometimes. When I don't get my way, I can be selfish and childish and pigheaded. I know I've said some pretty stupid things today. I wish I could take them back. (really hurting)
But even if it's too late for that, there's one thing you should know... I love you, Keiko. I always have, and I always will. (a beat)
Anyway, I just want you to know I've left a letter of resignation on Commander Sisko's desk. I'm ready to move down to Bajor tomorrow, if you'll have me. And after that if you want us to move back to Earth that's okay too. I'll do whatever it takes... I just don't want to lose you.
It's such a sweet scene and it shows, when push comes to shove, that O'Brien will do almost anything to make it up to Keiko. Those are the words of a man grovelling to his wife and begging for her forgiveness. It scenes like that which make the Keiko/O'Brien pairing so sweet and powerful. It's why I love both these characters because they're treated not in a soap opera way, but instead confronted with very real issues that many couples face: moving home, changing jobs, who will take care of the children.
If you don't like that, that's fine. But I'm merely trying to explain what is so great about Keiko (and O'Brien).