OCS is nothing compared to BUD/S. There were two guys in my class who went to BUD/S. Both rocked PT at OCS and maxxed out everything. One made it through BUD/S and is a SEAL, the other guy broke his leg twice and got sent to SWOS.
Also, keep in mind that if you decide to enlist in order to get to BUD/S, you can still attrite, and then you're left with serving the remainder of your enlistment. If you're going to be happy working some random enlisted rate if you fail, then have at it. If not, then I wouldn't recommend going that route. What's the worst that can happen if you put SEAL on your dream sheet at the officer recruiter's office? You get it. If you don't even try, though, you'll never get it.
You can also try calling Bupers and seeing how many specwar officer slots there are for the year so you know what you're up against. So what if there are only 10 for the year. There were only 10 slots for lateral transfer to NFO, and I got one of them. Hey...someone has to do it, might as well be you.