IIRC, Good Low is now the passing standard at OCS, so you need to hit that regardless. If you have down time throughout the day, just bang out a few pushups or hold a plank (both with perfect form, of course) and you’ll get stronger over time. The best time to start getting in shape was yesterday; the second best time is now.
Edit: Your performance will probably drop during the IST bc it’s conducted at 0515 on little sleep and without a warmup. Before heading to OCS, you should be hitting Good High easily, potentially into Excellent.
Don’t expect OCS to get you in shape, it’s expected that you show up already in good shape and you will definitely have a tougher time if you don’t. You definitely will get in shape at OCS but YMMV