It doesn't matter if it's an award ticket booked with DL miles. The fare codes/classes are established by the operating/marketing carrier. They might not be O for business class award seats on all partners, although there was an attempt to standardize this *somewhat* several years ago. In fact, I've had partner award tickets in business class using DL miles that were not booked in O for certain partners.

IME DL agents don't necessarily see or know how to find partner PNRs. It's better to ask the partner airline operating carrier directly (and then try to verify it on their website), especially since it's good to call the partner anyway to verify that all looks OK with what they see as a reservation and matching ticket.