Yes you need an OC. A head coach is going to spend time with the offense, defense, and special teams. They need that guy who is doing the grunt work monday through saturday for all three aspects. A head coach is coaching up the coaches just as much if not more than they are players. You need the OC to develop the initial game plans, run the installs in practice, etc. The HC needs to sign off on everything, and they will instruct the coordinators before the summer what type of schemes they want them to build the offense/defense around, but no one has the time to play both HC and full time coordinator. Play calling is just the smart part of an coordinators job that we see and can be critical of on a weekly basis. What happens though the week is going to be way more important than calling plays.