But then after you have received the OWP you are no longer on implied status and you need to extend your stay? or are you allowed to stay until your OWP expires or a decision is made on your case?Not exactly. The simple fact of applying for spousal sponsorship does not extend your status, you should extend it to maintain your legal status in Canada. However, applying for an OWP with your spousal sponsorship application does extend your legal status (as long as you still had legal status when CIC received your application). Between the time your current status expires until CIC makes a decision on the OWP, you'll be on an implied status as a visitor.
This is confusing, if you have an OWP it would seem impossible to extended a visitor visa as you are no long on the visitor visa.