Think of the OHSQ as a BSQ with the bar elevated a foot or so higher. The horizontal position of the bar relative to your body in the bottom position ends up being essentially the same as it is held in the BSQ. Consequently, if you have mechanical errors in your BSQ, they’re going to show immediately in the OHSQ. Fix them if they crop up.
(Obviously I’m referring to a weightlifting style BSQ – relatively high bar, deep, not wide stance.)
If you can’t sit comfortably in the bottom position, there’s a problem with flexibility somewhere along the line. Work band or broomstick dislocates for the shoulders, stretch the chest, keep the ankles flexible and the legs limber. Holding the bottom position with a moderately heavy bar overhead for time after your workouts is a good way to actively stretch the movement; make sure not to relax anything while sitting there.
Once you have the flexibility, the movement is almost trivial to perform (provided you also understand how to lock out a weight overhead). You should be able to progress rapidly to heavy weights. In the long run, working snatch balances / drop snatches is probably a better exercise than the OHSQ itself.
(On the other hand, if you don’t have the flexibility, the difficulty of the movement is greatly increased, which is where the misconception that the movement is actually as difficult as most people make it out to be comes from.)