Welcome!
Yes, the OCS application has 3 slots, so you can apply to a maximum of 3 designators at one time. Using this can be beneficial by casting a wider net. Pilot/NFO is a perfect example as it's the same board. Let's say you had #1 Pilot and #2 NFO, there's a chance that even if you don't select Pilot, you can get selected for NFO. However, if you only had #1 Pilot and you weren't selected, you don't have the additional consideration. (Not to say Pilot or NFO is better than the other, just an example).
The flip side to this is:
Very different designators on the same application (for example Supply, Intel, Pilot), could be viewed negatively as you don't really know what you're interested in and nobody wants to be 2nd choice. (Not something I have heard confirmed by a board member, but have heard from several mentors during my application process)
From what I have read recently, if the designators you're applying for aren't on the same board, they are sent to the boards in the order of your preference. (If your #1 board hasn't released results, you're possibly not even in consideration for your other options yet)
If currently active duty, it's best practice to have one of your interview appraisals conducted by someone within the designator you're applying for, multiple designators of interest make this a bit more complicated.
Letters of recommendation are also an option, but I think each designator might have differing opinions on their usefulness to evaluate an applicant. I would always recommend taking the time to get at least 2-3 that highlight specific work related to your designator of interest or specific skills that might be beneficial to your role as a naval officer (leadership and management skills come to mind).
I believe you've already submitted your application based on your post, but maybe this information will help you assist some of your enlisted service members interested in commissioning in the future.
