What’s the difference between being a US Navy Department Head or an Officer in Charge?

A 'Department Head' is a position almost entirely exclusive to a ship. You are the officer that is the head of a particular department-- Operations, Engineering, Supply, etc. Different sized ships have different numbers (and ranks) of DH's. The very largest ships (the Aircraft Carriers) have O6, post-command officers as Department Heads for some departments. Most of the smaller ships DH's are O3-O4.

On shore commands, the people who do the exact same level of job as a Department Head on a ship are referred to as 'directorate heads' or something similar to that. We affectionately refer to ourselves as 'bubbas' most of the time.

Your levels of officers on a ship are Division Officers, Department Heads, then XO then CO.
Your basic levels of officers on staff duty are 'Action Officers', 'Directorate heads', and OICs (with some Chiefs of Staff, deputy OIC, other randomness thrown in depending on the staff.)

An OIC is a 'boss' that isn't a 'Commanding Officer'. Not exactly tied to only ships, but predominantly ships and fleets, a 'Commanding Officer' is (wait for it) in command of something. Most shore units don't have 'Commanding Officers'... they have 'Officers in Charge' or OIC's which do the exact same thing as CO's, but are 'in charge' of something rather than 'in command' of something. It is as semantic an argument as it can get with some potential ramifications regarding what types and levels of punishment can be wielded by an OIC vs. a CO... For all practical purposes-- the difference is semantic.

As to your specific question: differentiating between Department heads and OICs...
Apples and Oranges as one applies to sea duty and the other only applies to shore duty.
Depending on the size of the department or shore command-- ranks can vary across the board. I've seen everything from O6 Department Heads to O2 OIC (usually in charge of 'detachments' of small craft).

If none of this makes anything any more clear, and instead only leads to more confusion, please take comfort in the fact that you are a member of a very, very large club. I think the Navy does it on purpose... Just like we can't do rank like everyone else.

As an E-8, I replaced a CWO-4 as OIC NAVCOMMU Message Center, Cheltenham, MD. On board ship/submarine, despite being a Master Chief Petty Officer, the closest I could be to a Commissioned Officer status was a boatload of “O” Collateral duties.

In the Navy OICs are designated because the job is not significant enough to warrant a CO. For instance as an O-3 I had orders to San Antonio MEPS as the OIC. I would be in charge and have two other officers reporting to me AF, USA to round out the group of URL needed to swear in new recruits. Yet there are still "early command" opportunities where you can be a CO prior to reaching O-5. Big difference is some Title 10 stuff and NJP.