If I complete the BOSIET and MIST training will I be guaranteed a job offshore?

Completion of any training does not guarantee a job.  Seeking employment often depends on previous experience. Some oil companies will recruit before training has been completed so being persistent is probably the key to success.  It is a good idea to register with an employment agency or seek advice from an employer, both can put you forward for relevant roles and give you advice on your suitability.