I thought you could remove the baseplate of AR15 mags by pushing through the small hole on the bottom to lift some spring detent. I have a bunch of OKay mags but I have jabbed a tool through that hold and it hits nothing.

What am I doing wrong?

The two little indents on the floorplate is what holds it on. I take the floorplates  off by slipping a small part of a thin screw driver like the larger size one of the  precision screw drivers sets you buy into the gap where the bent down (down because the mag is upside down when I take them off) piece of the floor plate meets the mag body. I give it a little flip and if lifts the indents up and prys the plate slightly off enough to pull it off with my fingers.