I was in the same situation as you. I raised my right hand before I even graduated. Direct Commission. Planned to stay in for 3 years and get the He*L out, especially after completing my 2nd year of active duty in Iraq. But that changed me. Decided to stay in after that experience and was offered a government paid tour in Germany! Thought I was getting out after this tour, and then got accepted to the FNP program. So, I can't complain so far. After school and its commitment, I will lock in 13 years, and might as well finish the 7 years to get 50% retirement pay for as long as I'm living.
To answer your question, yes you still can! well, online nursing schools anyway, and also have military tuition assistance pay for it! but you will owe 2 years as an officer after your last class. So if you applied for military tuition assistance to pay for your tuition assistance qualified MSN program, on your first year of active duty, you will not owe any service back since it's concurrent with your 5 year contract. If you finish your MSN within 2-3 years.