I’ve seen and been taught many ways of controlling the opponent while going for an omoplata. The most common I see is to grab the belt. Depending on the setup, some have you hug the knee/thigh to your shoulder. Eddie teaches the jiu-claw, hugging behind both thighs.
They’ve all worked okay for me, but lately I’ve been doing this, after seeing my instructor do it a lot and having him show it to me:

I am grabbing the end of the pant leg and straightening my arm to stretch it up and away. I keep scooting my hips away and making my leg heavy, like I want to touch my hamstring to the ground, to pressure their shoulder down.
If they try to lift their leg and step over me, I just keep stretching their leg up and away. Every time I do this, they are forced to go flatter and flatter, taking away their base.
If they try to roll forward, I just keep stretching their leg back and away, and they can’t.
Once I feel I’ve got them broken down enough (e.g. no posture, almost on their belly, shoulder on the floor), I’ll sit up and start grabbing the belt or hips, climbing up the back and getting upper body control, like the haress grip or grabbing the far lapel. Then I arrange my legs and sit up and drive forward to finish.
You can try this no-gi by grabbing the ankle, but like all no-gi grips like this, it’s not as strong.
Give it a try and let me know how it goes.

This crap is fake. Doesn’t work.
:)
hes right it is fake that guy your holding could have gotten out if he wanted to
I tried this last night and it worked like a charm, although the guy I tapped out was different from the guys I normally have trouble tapping out with an omoplata, so you could argue that my method isn’t all that scientific. He was a blue belt, though, so…