Agree that a new OC is a #1 necessity and priority before the NFL draft. The designer of plays and the play calling against a particular team, during a game, is very, very important, as important as the execution of those plays. A poorly designed play with the best execution will usually fail, IMO.
The Dolphins need to hire a person with talent in both areas. The "guy" doesn't have to be a "young guy" or an "old guy" with an abundance of experience, it has to be the "right" guy that recognizes the current player talent, can design plays for that talent and call plays he believes will be tactical during the game so the team can score, repeatedly. That "right" guy should also have the confidence of the players. As for who is the "right guy," I haven't a clue as I don't follow the OCs.
Where would I find the next OC? I'd start looking at the teams that are scoring machines to assess their OCs and look at what makes that team offense so good. That would be my starting point. Information gathered should lead to the next points to assess and explore.
I do believe there is a "right guy" out there.