At OCS, you are technically in the USNR. When you memorize your chain of command, it even says "United States Naval Reserve" for Section Leader. Retirement countdown starts on commissioning day.
Active duty time is active duty time. It doesn't matter if it is USN or USNR. There is a reason they watch SELRES active duty time closely. It's to keep reservists from getting an active duty retirement.
By your profile, you've have served a day yet. Why don't you wait until you actually have just a little bit of experience to back up your proclamations.
All my Army Reserve and Navy Reserve active duty time including my time at AOCS counted day for day toward my retirement.
Service as a Midshipman is specifically prohibited from counting toward retirement. But this term apply to those attending the Navy Academy or ROTC, not OCS. It is also due to the fact the Navy is paying for your college education in these programs (or potentially paying with ROTC).
When you sign your OCS contract, you enlist in the USNR With the rate of Officer Candidate (OC) and paygrade of E-5. When you go to OCS you are a USNR OC3 on active duty. It counts toward retirement.
Another different between Midshipmen and OCs is that your PEBD (pay longevity) starts the day you signed your OCS enlistment contract. For Midshipmen it starts the day they are commissioned.
One of the good deals about the old BDCP is that they were on active duty as USNR OCSN to OC2 while finishing college. Then they remained on active duty until finishing OCS. By the time they commissioned, most only had about 18 years to go toward retirement. It was the ultimate good deal (unless you didn't make it through OCS, then you remained on active duty enlisted for a few more years....)