Ok, this ended up being long. I've put on my bartender hat...and take all this with a grain of salt. As for any of the posts you read...remember that you're talking to a majority male audience with Type-A personalities who love flying and the Navy...so don't go telling your boy..."This guy on AW said..." because that's NOT a good thing for anyone.
Foremost...do what you think you should do...because that's most likely the right thing to do...
The addage: "The hardest thing to do is ususally the right thing to do." hold a lot of water sometimes. That's true with either applying to OCS against your fiance's wishes OR turning OCS down and doing whatever else you do.
My fiance and I are set to get married in January after I graduate from college in December.
I'm going to go against the crowd and say good on you. Especially if you have been together for a few years and love the guy because I'm a romantic and love can indeed conquer all. (cue violins and probably negative rep pts

)
I've decided to apply to OCS and he's not happy with my decision at all.
It's a great thing that you applied...but you said so yourself..."you've "decided. This is most likely one of the things that he didn't feel he had a say in...and it frustrates him.
His main concern is that I'm going to "unintentionally cheat" on him because "everyone in the military cheats on their significant other." ummm...I highly doubt that 100% true.
If it is true, than they aren't quite divorcing at the same rate as your fellow Americans...also, I've been deployed twice...The number of married officers that I've been with have never "unintentionally/intentionally" cheat on their spouses. There are horror stories out there, but rare.
Link I found on military divorce rates.
He said that he supports my decision but thinks that it's a "dumb idea" that I've made on a whim.
That's good that he states he supports your decision, but then it's not good that he calls it "dumb." (this is the point, btw, that WE all get offended here and instantly shut down any sort of liking of your fiance...because than we are indirectly being called "dumb" by him...) Did you talk to him before about this? Of course if you never said anything before just now and you've been together for a few years, than I can see how he calls it a "whim"...but I don't know how long you all have been together. Just looking at things from his POV.
He ignores the fact that I've wanted to be in the military since my brother came back from the Marines years ago.
I would disagree...as if this is true...than it's a fact. We live in Denial of facts, but we don't ignore them. Try not to shut down when you feel he's ignoring this fact...just realize that it's something that supports your decision to join...and he doesn't like it. So..he denies, in his head, that it's true. It ain't just a river in Egypt.
He's 37 and I'm 21, which obviously is playing into his insecurities.
Ok, so some people don't feel that this is normal...and don't take offense to this...but what about your insecurities...how did you end up with such an older guy? The rest of the people on this board are focusing on his age...but how did you get with him? Is he a nice older guy? Did you meet at church (an ok place, IMHO, to meet an older guy) or Kids N' Squids night at the local bar (not such a great place)?
Age isn't a big factor if you are both ready for it...but you are a child of the mid to late 80s and he was driving at that time...there are definite culture differences. Your favorite rapper growing up was
this guy...he didn't know what rap was.
I want his help on my application and just be able to talk to him about military life since he was in the Army. OCS is my dream and if I'm accepted the benefits for us will be great. He's not seeing this.
Army V. Navy...officer V. enlisted...HUGE difference as mentioned before...especially if he was enlisted...make sure that you tell him your dream...and if he's the right guy...
I just want to know that it's not a bad thing for wanting to pursue a dream....
And this is the key. Yes it is a bad thing to pursue a dream, IF, it means that another dream that you wanted more would be possibly ruined...IE...you go to OCS...he can't handle it or you can't handle his criticism, whatever, and you break it off. It was your dream to be married to this guy too (I am guessing since you said "Yes" when he asked). What dream did you want to come true more?
I wish you luck and let us know how it turns out. Either way, I've got a beer and a wife you can talk to if you need it.
~Dan