Depends on the test you do before enlistment. Once inside, your performance is based on platoon level. Say your platoon has 60 people. You will be ranked 1 to 60. Usually 10% top will go ocs then subsequent go scs. Depends on the intake also.How will you be ranked ? See your commanders like you anot and backed by ippt , sit test and must have gone through all high key. Other optional criteria can be set by your very own commanders. My pc was a jc kid , and my 2 sect comms were very high gpa. Naturally, they see the transcript/gpa which you submit while in bmt. My friend gpa 1.8 became an officer. My gpa of 2.8 will better ippt score cannot. So it very much depends on your own commanders and sots available. Those that are not inclined doing well , will naturally choose someone they think can lead.
Express interest is see whether you interested or not , dont care what scs or ocs. I put interested in ocs , not interest in scs. Still went to scs.