Col Chase told us right after the 5 mile Moto run, "Congratulations, you are mow Marines, you've earned it." So according to the O-6 you are a Marine by title.
Now, what do you call yourself before you commission? I was NROTC so we were still referred to as 1/C. The MECEP guys still went by their rank. As for the PLC guys that I knew on campus that went to OCS with us that summer, just called each other 'Marine'.
You're not going to be filling out any paperwork - that I know of, you are not going to get a CAC ID (hard card), you might wear your MARPATs which will have your name and "U.S. Marines" so dont worry about any specific rank. You're a Marine. You will be a 2nd Lt soon enough. It's a weird limbo period but who gives a shit, at that point you will be so happy that OCS is over that it wont matter.