The OU is not pro-military in the sense that it gives the military special consideration. The OU is very even handed and is pro everyone that tries to study with the OU.
All the tutors I have had have been extremely helpful, military or not.
All the OU requires is not that you do the best but that you do your best.
While the OU does all it can you must ensure you get as much help as the military can give too. Make sure that you are not rostered when tutorials or study periods are scheduled. That always used to get my pip as a young whippersnapper - oh, he's at college today. Now I have the maturity to realise that that time-off at college was beneficial not only to the individual but to the service and the country at large.
Go for it. The 60 pts per year committment, typically 7 assignments and an exam, can seem quite daunting but provided you can get stuck in it soon passes. The OU says 16 hrs per week - for some people and some courses you may get away with considerably less and also not work every week. On others you may feel you are working full time.
What line of study have you in mind? The social science courses I have done have very little external reading and no books to buy. The history courses are very reading intensive. For one TMA I had to buy about 6 books but that was exceptional.