NCR:

If I read this right, you already have a Bachelor's Degree. If so, ROTC is not an option. Neither is the Academy.

The only option for those pursuing the USAF as an officer, post-college, is OTS. If you want to go Air Guard or Reserve, they have a similar OTS system.

Pilot slots are primary given to the Academy and ROTC; usually, OTS fills the missing spaces (guys who can't pass the medical, change their minds about flying, etc). Sometimes OTS has lots of slots; other times, not so much.

Active-duty flying training commitments are, I believe, universal for the Air Force, Navy, and Marines, at 10 years. I do not know about the Army and helicopters.

For the Guard and Reserves, they can't tie you down as securely to 10 years (since the members are usually part-timers), but they make it so if you try to leave early, you will never get another military flying job....nor a positive recommendation for a civil flying job.

Since you have your heart set on flying a tanker, I would pursue a Guard or Reserve unit that flies them. Keep in mind, saying "I only want to join to fly, nothing else" will not weigh favorably. They are looking for people to support the mission, not an obvious attempt to get some heavy time then bail for something else.

Last thought: when I joined, I wanted to fly 4-engine heavies and do minimum time to go to an airline job. A funny thing happened along the way: I found out I loved yanking and banking and flying upside-down. I felt pride in my uniform and serving my country. I still got an airline job.

And I'm back in the Air Force, yanking and banking, because it is still more fun (and a better-paying job) than the airlines.

Good luck.