I am a senior and have a 3.3 GPA for a Biology degree. How likely am I to get into Navy OCS?

Don't focus on one Service unless you have a very strong reason to do so such as a long-standing family tradition of serving in one Service. Talk with officer recruiters from all 5 armed services (Army, Navy, Marines, Coast Guard, and Air Force). Determine which Service best fits your personal goals and personality. There are many variables in the value of each Service's offer. BTW, as a biology major, you might want to consider another Uniformed Service, the National Health Service. Look them up on the web. They have some very interesting missions.

Examples to consider:
-Accession date. Will you have to wait a few weeks or several months before you enter the Service?
-Pathway to commission: Will you be expected to complete the Service's basic training course and then move to Officer Training? Will you go directly into Officer Training?
-Rank. Most Services will bring you in as an O-1, but some might consider you entering as a Warrant Officer. Which will best suit your plans for your personal future?
-Training Opportunities and Service Commitment: Based on your personal goals decide which Service offers training that matches those goals. Next, determine the length of time you will be required to serve as a consequence of attending the training.
-First Assignments and Deployability: Services may be willing to promise assignment into designated specialties after completion of officer training. Are these guarantees consistent with furthering your life goals and objectives? What is the likelihood of deployment shortly after completing training in the offered specialties? What is the recruiter willing to put in writing?

Once you carefully weigh all of the factors in each Service's offer, then make a commitment to serve.

Really depends on what you're applying for. Any idea?