Not a regular here but you have to check with MINDEF about career advancement.

1. You must make sure you can graduate from OCS even before thinking about life as a commissioned officer.

2. As you said, those who signed on early are usually a MAJ at this age especially with a degree. If you were to commission even as an LTA, you are still losing out unless you can get good appraisals.

3. You also need to think about the scenario in which you leave service before or at end of the contract. As far as I know, it's not easy to switch from uniformed service to the private sector. Most of those I know who left service after 5-8 years usually will join back the civil service.