When you enlist in the USAR or ARNG you may choose an enlistment period of up to 6 years. However, as soon as you graduate college and get commissioned through ROTC (usually on the same day as graduation), your enlistment contract is terminated—no matter how much time is left on your enlistment.
Upon commissioning, you start a brand new career as an Army officer. That means that even though you were enlisted in the USAR or ARNG while going to school, you have the option of serving as an officer either full-time on Active Duty, or staying part-time as an officer in the USAR or ARNG.
Also, the job specialty (MOS) you had while enlisted has no bearing on your career options as an officer. If you like the field you were in as an enlisted soldier, you can choose that field as an officer. If not, you can pick any of the other 16 basic branches.
SMP does not lock you into--or out of--any commissioning options. You can still go on Active Duty, or switch from having been enlisted in the USAR to an officer in the ARNG (or vice versa). You can stay in your enlisted career field, or switch to something totally different