I recently spent an afternoon getting the original Oblivion to work optimally on Steam Deck. Despite being "playable" out of the box, it's kinda not: the vanilla PC version of Oblivion lacks gamepad support, meaning gamepad glyphs aren't present. While the Steam Deck's community controller layouts tab is a godsend for situations like this, it still means having to adapt to the disconnect between keyboard or mouse prompts and the Steam Deck's Xbox-style glyphs...
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Oblivion Remastered is 'verified' for Steam Deck, but the original with mods is a far superior handheld experience
Source: PC Gamer (opens in a new tab)