If you have an OWP application that has been submitted, not returned, but still pending approval, you are granted “implied status.” This is as good and as legal as a valid Visitor’s status. This will prevent the need to keep applying for Visitor’s Extensions. The only grey area is if you’re truly worried that your OWP app. may be returned and your legal status is about to expire. In that case you may want to apply for a Visitor’s Extension to cover yourself. Because should the OWP app be returned, then you’d be out of status.
On a related note, I read in other threads that Visitor’s Extensions are rarely denied if you have a PR application pending, so I wouldn’t worry about a second request being denied. You just have to mention the PR app. on a Visitor’s Extension application, should you decide to file one.
Hope this helps … sorry, I know it can get confusing when you’re trying to apply for PR and stay on the good side of the government!