OCTP was for entry directly as an officer (restricted to combat arms, I believe), without a degree. I‘m not sure whether it exists today -- several leadership reviews have come out strongly in favour of requiring all officers to have degrees. Indeed, there is growing support here - and evidenced to be in practice in the US - for post-grad for senior officers.
I would think that if your high school marks were poor, the best way to improve your selection options would be to take some highly relevant college courses - university courses if you can get admitted - and to achieve excellent marks on them. This will show dedication to the cause.
The other element in the plan would be to take any and all officer professional development courses that you can - used to be called OPDP (Officer Professional Development Program). I assume these exist, or some comparable replacement. Take them in order, one at a time, and make sure you do well.
Together, these two academic options can go a long way to erasing youthful mistakes and showing you are prepared for the rigours of UTPM (i.e., attending university as an officer cadet after having been in the ranks).
Additionally, you want to show that you have leadership potential - volunteer for leadership opportunities, get your courses, take Jr Ldr, etc.
IMHO, this is the route for an NCM to become an officer, and possibly the only way for a civilian to become one if they can‘t get selected directly. It is a difficult path to take, and there is no guarantee that you will ever be commissioned.
If you can be selected directly, and you know you want to be an officer, I think it is much better to do so, rather than doing a few years in the ranks first. Be aware, though, that poor high school marks probably undermine recruitment under ROTP - the plan you would attend RMC under.
