If you don't have experience in oil field and still want to start your offshore career you should pursue for entry level positions. These are commonly: roustabout (general worker),roughneck, cleaner, painter, deckhand, wiper. In catering service entry level positions are: galley (kitchen) hands and bedroom hands.
Even if you are strongly skilled but without offshore experience you should consider applying for entry level position because there is much difference between doing some job on oil rig and doing the same job onshore in some other field.
It is very important to become familiar with working offshore and get some offshore experience. When you start working and get acquainted with working on the rig there will be opportunities for promotion, for sure. Of course, if you want to learn and to move up the ladder. And when you already have offshore experience it would be much easier for you to find your next offshore job.
For all positions except entry level offshore companies rather hire people with previous offshore experience and some of them hire only entry levels and then do promotions within.