If you were on a student visa immediately before being on the current bridging visa, then it OSHC would be the insurance you need. In case you are not sure, as for how long you had need an insurance for, you can purchase a cover for 1-year validity. You can always cancel this policy and a get a refund on the term period left of your OSHC policy, if you get your new visa before the OSHC expires. In the case that you need a 485 visa, you will need a special type of insurance in order to prove that you have a health insurance at all. You will have to see whether an OVHC policy covers it or not, because all of them don't do.