The person you've spoken to at GWR is incorrect - they evidently don't understand how time restrictions work in railway ticketing. The applicable restrictions depend on the ticket you hold and the details of its restriction code; trains aren't blanket "peak" or "off-peak" for all tickets.
You are proposing to buy a London Terminals to Oxford Off-Peak Return, which has restriction code FP. As you rightly say, the text for the outward journey on this restriction code doesn't mention any evening peak restrictions. Therefore, you're free to take any service after 09:30 (plus the 09:23, which is specifically permitted if travelling to Oxford under code FP).
The reason why you're not being offered the Off-Peak Return by journey planners is that GWR have contradicted the written restrictions in the industry data that journey planners use. Here they have included evening peak restrictions from Paddington, on the outward journey for FP, despite these not being mentioned in the text. Such contradictions are common, especially with GWR, who continue to base evening restrictions on individual departures from Paddington - leading to people being overcharged. But you are entitled to rely on the contents of the human-readable text of the restriction code. Helpfully, this is linked to on the ticket, so it is easy to demonstrate what restrictions actually apply.
I would prepare for potential difficulties at the Paddington barriers and/or on the train though. Many of the staff may wrongly believe there to be blanket "peak" restrictions in the evening - or even if they do understand that restrictions depend on the ticket held, they may think an Off-Peak ticket to Oxford has evening restrictions. We have received numerous complaints about the Paddington gateline staff over the years in this regard, since there is such variability in the restrictions that apply.
If you are overcharged or denied boarding, you should contact GWR afterwards to complain and seek redress (whether that's a refund of anything wrongly charged, or Delay Repay if you are denied the opportunity to take a train you should have been able to take).
