He's going to call plays, but I do think he needs the right guy as his OC so that he can turn over a lot of the daily game planning duties to the OC so that he can handle some of the HC stuff. I think lots of guys fail because they spend too much time on their side of the ball and neglect the HC part of the job. Payton was a master of it in part because he could trust Pete with a lot of the day to day OC stuff.
But sure, the DC hire will be vital and likely more important.