Oil Rig jobs fall under three categories
Drilling - Begin as Roustabout use muscle more than brains. Carrer progression to Top Management through acquired learning, consistent good performance and attitude.
Engineering Technical - Responsible for operation and maintenance of Drilling and Support Equipment. May enter as trainee but requires mechanical technical aptitude to advance to Maintenance or Engineering Supervisor.
Support Personel - Catering, can include room cleaning and laundry, and Marine, Deck personnel. The latter pays better and requires a 'Z Card', issued by USCG, for entry. Type of the ladder is Barge Engineer or Captain.
There are three basic entry points to land or offshore Drilling rigs.
Rig Hand - Working for the Drilling Contractor.
If totally unskilled entry is as Roustabout. Very difficult to get unless you know someone with influence. If you are in an area where land rigs are operating you might have some success if you can get to know the crew in their off time. You might also try Century Resources, Century Drilling in Australia. They have high turnover and may be able to sponsor a work visa.
Skilled or semi skilled with some training or qualifications in; computers, mechanical, electric, hydraulic or welding you may be able to enter training programs. Pride is the largest in total rig count. Transocean GSF is the largest offshore.
Service Company - Support companies.
This varies from Catering to Mud Logging (Well Bore Fluid analysis) to Tubular inspection and running. I would suggest you visit 'Rigzone' for a list and contacts.
OEM - Capital Equipment Manufacturers
The primary OEM is NOV, National Oilwell Varco. They hire various skill levels to assist in installation and service of rig equipment with training supplied. You would need to be based near their facility. When you are qualified it's pretty easy to switch to a Drilling Contractor through contacts you will develop.
STAY AWAY FROM ANY OF THE COMPANIES OFFERING OILFIELD PLACEMENT!!!!!!!!!
Rigzone is the only source of job opportunities that is not in it for the money. They are a reliable source of information and have a section dedicated to jobs.
Any offering entry level opportunity without training is a scam.
I hope this helps. Mail me if you have any further questions.