The problem I think is, Vlad is probably playing, but Oksana isn't. She doesn't want to play, and has never really been the type of cat that played a lot with others. I think she may have soured on playing with others because Vlad was always a little too rough with her as kittens.
She does now and then go up to him and they do the nose touch thing, and she licks his face, but if he tries to play more or lick her face her ears go flat and she backs away.
It's only been in the last 4 or 5 months, that he's taken to sort of ambushing her. I've even caught him waiting for her next to the litter box. I don't allow him to wait for her but I'm sure while I'm gone if she goes to the litter box she's likely been ambushed coming out. Which makes me think she may have gotten her cystitis because she wasn't comfortable using the litter during the day.
Now I figure, at least if she's in my bathroom during the day she can use the litter all she wants and not have to worry about being attacked.
I've also got to say Vlad is an equal opportunity attacker. However, the others handle it differently. Maks submits to him, so Vlad plays more than anything. Kiska gets on her back, swats at him with her paws and defends herself so he gets bored and leaves her alone. And Czari,
well, lets just say I've seen Vlad take a couple of steps back when Czari voices her disapproval.