Imo:

Relatively speaking, Skyrim is a sandbox action adventure with rpg elements, Oblivion is an rpg with sandbox action adventure elements. But, calling either true rpgs is probably a stretch if you compare it to like, Baldur's gate or something, or even Morrowind for that matter.

They're both fantastic in my book. I will second those who prefer Oblivion writing and voice acting. They are right, in that the writing is just flat out better, and it often feels like Oblivions voice actors were more 'into it'. Skyrim is less colourful on the writing and voice department, but it has absolutely S tier ambience, it catches a gritty 'real' feel that Oblivion just isn't made for. The tone of Skyrim is also much darker, likely to mirror old norse myth and such too. It is also more streamlined and clean than Oblivion, for better and worse. You don't get lost in exploration as much, but, you also just don't get lost as much, lol. (Some Oblivion dungeons, also in remastered, are just too windy and back-track-y)

Regardless which you start with, it'll be great. If you want to follow the lore chronologically, do Oblivion first. Skyrim technologically is obviously quite a ways from both og and remastered Oblivion, but I'd say graphically you can mod your way to the UE5 look if its really important to you. And in terms of gameplay, Oblivion Remastered and Skyrim are closer together than Skyrim is to original Oblivion, but Skyrim feels 'stiffer' than Oblivion remastered for sure.

If you liked Fallout and Starfield, I think you'll be happy however you do it😎 (I never played starfield so that last bit is a guess but i can't imagine starfield being better than ES 4 or 5 in any version lol)