I was both a USAF SSgt & Captain. I attended BMTS when I enlisted, but never attended OTS.
CeciliaM covered it for you. I came in Direct Commission O-3, due to my status as a Psychologist. I never attended OTS and was a Captain the second I was sworn in at Sheppard AFB. I was prior service during Medical Officer Orientation, so I knew most of the stuff already, but the NOOBS were priceless. One guy (a DO from Des Moines) had been ROTC as an undergraduate...and he actually put his ROTC ribbons on his Class A. I don't think he ever lived it down.
Oh yeah...and we partied down in our orientation...and you haven't partied until you have partied with the Med. Folk. LOL We did, however, attend classes and march around in an aimless fashion.
The difference, as Cecilia said is that instead of ordering us around, the trainers would say "Do this, Sir/Ma'am." We had rank Day 1 and were Officers.
Line officers attend OTS/OCS depending on the service. In some cases, people attend both Basic and OTS/OCS, in others, the officers only attend OTS/OCS. OTS/OCS is the real deal...and is quite demanding, physically and mentally. Remember, there you are an Officer Candidate, not an Officer.
Hopefully, this didn't make you more confused than you started out. LMAO