Help with OCS Application - Letters of Recommendation

What is the minimum level of authority that will be acceptable as a letter of recommendation? Being enlisted, I don't have very much contact with officers over Captain (O-3). What are the best ways to request letters of recommendation? I could request letters from my past college professors, but I don't know what is the best standard practice.

I am in the very beginning stages of an OCS packet. I am a drilling reservist and have been conditionally released for active duty. The new fiscal year is just around the corner, so more OCS slots will be available according to the recruiter I'm working with. At this time I am looking for a career change after 5 years in the Army Reserve. Since I already have my degree, and am not interested in more schooling, ROTC is not an option for me.