How hard is it to go from USMC NCO to US Army Officer?

I didn't move over from NCO to officer, but I did move over from active duty USMC Officer to active duty Army Officer in order to serve in SOCOM (MARSOC did not exist at the time). If the career opportunity was available for me in the Marines at the time, I would have stayed in. However, many Marines switch over to the Army for broader career opportunities.

Switching over was a significant culture shock for sure, but I adjusted. The Army is much larger, and there is a lot more variety within it. In the Marines, people associate with the Marines. In the Army, people associate more often with their unit or specialty. The Army is just so large, it can't have the same bond that Marines do. However, if you go into a career field that you enjoy, you can can benefit from some of that camaraderie within your field (though it will probably never match the Marines).

The Marines tend to pride themselves more on discipline, customs, camaraderie, and being combat focused (every Marine a rifleman). The Army is much more diverse... a much bigger tent if you will. You have a lot more kinds of people in the Army. There are plenty of great officers, NCOs, and Soldiers in the Army. If you have an opportunity to transfer into a career field or career track that you are excited about, and cannot otherwise do so in the Marines, you will probably not regret it. Just be ready for some cultural changes.

As a Joint Officer, I work extensively with all the services. From my perspective, as a Marine Joint Officer, you will experience a cultural adjustment, but nothing too onerous. Just be certain to maintain a flexibility of mind. As has been stated by others, many Marines opt for the Army and the other services to broaden their opportunities. It's not a good thing or a bad thing, it's just a thing. The Army, overall, as a culture is quite doctrinaire, while the Marine Corps is more "entrepreneurial". There's an old joke that lays this out pretty well. Two LtCols were talking at the Army War College, one was an Army 0-5, the Other a Marine 0-5. After listening to the Army 0-5 pontificate for an hour about the Army's doctrine, the Marine was asked what the Marine Corps' doctrine was. The Marine LtCol responded "we don't have any doctrine, we make it up when we get there":) No matter which branch you serve with, you'll always be a Marine. Good luck and smooth sailing.

SGT,
I changed from SGT active duty USMC (2BN 3D MAR, 3DMARDIV) to SGT National Guard reservist. The culture shock was quite big but I think most of it was because I worked with reservists. I did this for four years before doing the National Guard traditional OCS program (18 months, one weekend a month). You will learn to lose some of the discipline and language you learn in the USMC, but the general military "feeling" is the same. You will have the same pride and bond with Soldier's as you had with your Devil Dogs.

Also, to be quite honest, being an Officer is much different than being an NCO. Your adjustment will not be determined by if you were prior enlisted Marine, but your ability to become a leader. Don't let your pride hinder you. Keep an open mind and you will be fine.