Oman employment visa can be applied without cancelling your existing Bahrain Visa. Also there is an exception that medical report from the authorised medical centre (GAMCA) may not be required since you have Bahrain visa. Where as at the time of when you undergo for Oman MOH medical you need to get a medical from any authorised medical centre in Oman. Earlier there was practice of giving an undertaking by the employer at the time applying for visa for those who are having GCC visa, that the visa would be cancelled later. This practice is not being followed now. In your case there would't be any problem as your bahrain visa also expiring in 2013.
All the best,
Regards,
Gurumoorthy T