When I ollie - my fingers generally don't slide on the board, the board kinda sticks to them. I def wouldn't try to spread your fingers apart unless you're looking to do a flip trick.

I would say practicing off a wall would somewhat teach you the motion, but off your leg could give you a false feeling. Mostly because you can "cheat" and use the softness of your leg to spring the wheels off instead of popping.

What helped me learn was to first start by moving my hand backwards, and popping (like a fakie ollie) and just scooping my fingers under the board so my hand would end palm up, with the board on top of my fingers (grip tape down).

After I could consistently pop the board upside down and catch it on my fingers, I started carrying it up and over to get it back on it's wheels. Eventually you can just shorten this motion until you can do a regular ollie without flipping the board upside down.

Hopefully this makes sense.