First, there is no OS course. That’s an entry level rating with no experience or training requirements. Are you asking about STCW “Basic Training” that almost everyone onboard a commercial ship operating offshore or internationally needs? That’s different. Whether or not you need it, and whether or not you will be expected to have it before getting hired will depend on your career plans.
As far as the AB course, that’s a course that substitutes for the Coast Guard exam for able seaman. It has a time limit of one year. So I would recommend that you first apply for Able Seaman endorsement, and if your Coast Guard service is found sufficient, you will be approved to take the Coast Guard exam. Then you can go take the course, or decide if you’d rather take the Coast Guard exam. And before you can be approved for the exam, you will need to also qualify as a Lifeboatman, that will require a course. But I also suggest applying first. The National Maritime Center will identify everything you lack for the endorsement, that will include the lifeboatman course, but if it includes sea service as well, you might want to reconsider t=when you take the lifeboatman course. You may not need it right away.
As far as getting a job as Able Seaman, that may also involve more STCW stuff depending on your plans. If it’s on a vessel that needs to meet STCW, you’ll almost certainly need STCW endorsements as “Rating Forming Part of a Navigational Watch” (RFPNW) and probably “Able Seafarer-Deck.” These have sea time requirements. They do not have training requirements, but they do have an assessment requirement that is analogous to a military PQS. For RFPNW, if you have Coast Guard qualification (PQS) as Helmsman and Lookout, you will be good for the assessments for RFPNW.
Here’s some Coast Guard “NVICs” taht woill give you more infortmation on the STCW stuff I mentioned:
NVIC 4-14 (Lifeboatman/Proficiency in Survival Craft)
NVIC 6-14 (RFPNW)
NVIC 8-14 (Basic Training)
NVIC 14-14 (AS-D)