Yay! My partner has been approved for the OWP! What a relief. Now he can start stressing about getting a job instead of stressing about the OWP
I know this comes up from time to time, so I thought I'd post for anybody who might be in the same boat in the future. PA's visitor status expired at the end of September 2018. We applied to extend it at the beginning of September, and submitted the PR/OWP app in October 2018. At that time he was on implied visitor status. We heard back from IRCC about the visitor extension in December 2018 (it was approved until March 2019).
Implied visitor status counts as "legal status" for the OWP.