When I speak with Oengul War-Anvil, he is missing several dialog options. He doesn't mention Queen Freydis' Sword, and he doesn't seem to have the "What do you have for sale?" option anymore either. I've had this happen with multiple NPCs. Honestly, it's a bit fwustwating.

As far as the quest dialog - finish any other misc favor quests you currently have. NPCs generally won't offer you a favor quest (that's what these kind of side quests are) if you already have any similar ones active.

In regard to him missing the vendor dialog line - wait for 24 hours and try to notice what time the day changes over. If its nowhere near midnight that means your daily clock and date-change are out of sync which can cause scheduling problems with NPCs - ie they won't open shop at the proper time.

Are you on the PC or a console? There's an easy fix for that time/date sync problem if you are on the pc.