If you are on a bridging visa and your most recent visa, or the one you are going to apply for, is a student visa, then OSHC is the right insurance for you.
If you don't know the duration you need insurance for, simply purchase 1 year's insurance. If your new visa comes through, then you can cancel your policy and get a refund for the remaining term of your OSHC policy.
If you are looking to get a 485 visa and need to prove that you have health insurance, then there is a special type of insurance that you need to get. Some OVHC policies cover this, but not all. We have some of these on our website: 485insurance.com.au