Hey all, major lurker here, first time posting 🙂 !!!
I took the MCAT last April, and the computerized OAT early this month,(Change in career plans! 🙄 !), so here's my $.02.
Both tests have their moments, for me on the OAT it was when I realized I was down to the last 30 sec on the QR section and still had 8 more problems to do. I also found the Biology section to be tricky, because you either know it or you don't. On the MCAT sometimes things in the passage will jog your memory on a topic.
That being said, I would still say the MCAT is more difficult, because you need to know the background material, how to APPLY the material to a situation, and how to interpret figures and data. Also, the length of the MCAT is brutal, as many previous posters have mentioned. The computerized option for the OAT was so nice, and I have my scores already instead of 2.5 months later for the MCAT!
I did very well on both tests (370 and 34P respectively), but I studied for the MCAT, and only decided to take the OAT 2 weeks before I did so I didn't study at all. I found that all of the background material I learned for the MCAT was (mostly) still there, so studying for one will cover most of the other test as well.
I also taught the MCAT course for Kaplan, and the most important(and most overlooked) prep you can do for the MCAT specifically is... TAKE PRACTICE TESTS. Once you know the background stuff that you need for the OAT or the MCAT, the best thing you can do is get comfortable with the test so that you finish as much as possible as well as possible on test day. (btw, I am not trying to say go take the Kaplan course 😉 I think you can prep without paying $1600 if you are dedicated, just get some practice tests and take them like the real deal.)
Basically if you feel confident about the MCAT the OAT is no problem, but not the other way around!
Good Luck!!
Just my $.02