You can apply after you’ve received an update stating you’ve been Approved in Principle. That will come towards the end of the PR process though.
However, if you’ve applied for an OWP with your PR application, you can get OHIP coverage once you’ve got full time employment, lasting for 6 months or more. You’ll just need to take a copy of your employment contract, or a letter confirming your employment status from your employer, to a Service Ontario Office.