Hi DubDub. You are not alone, so don't let it deflate your ego. I coach the OL's to younger kids, and the onlykids I know that have the flexibility to do them right off the bat are the early and preteens. I've seen a ranked powerlifter, capable of 500 plus, not being able o do a full overhead squat with just the O bar. It takes lots of hip/ankle/shoulder flexibility. First off, get a PVC pipe, wooden dowel, or no more than a standard 7 foot bar (approx 18 lbs).
Try them with this. Yes, a snatch grip is used, and the bar should be just a couple of inches off your head. As you sit back for the squat, the bar is going to go slightly behind your head to maintain the weight over your center of gravity. Do lots of dowl or PVC dislocates to get your shoulders flexible enough to do this. The trunk will lean forward. As in any squat, maintain an arch in the lower back, with back flat, and I recommend, as in any squat, keep the chest puffed out and squeeze in the scapula. Do them everyday, along with front squats, with a dowel, as a stretching exercise. It will not cut into muscle recovery, but will give you the ROM needed, and will also stretch out your shoulder irdle. A big plus to these, is you have to work your trunk isometrically, and its a great core exercise. Go slow, get the flexibility, get your core shored up, and add weight gradually.
These tips are just off the top of my head, but I strongly suggest you register in at crossfit.com, and get on the boards over there. In the exercise board, there are numerous posts on the overhead squat. just get in and do a search on "overhead squats", and you will find many more tips from the USAW certified coaches (Coach Buergner being one of the best).
Good training, DubDub