 If you liked
Opus
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 ...then you might like
Yume Nikki
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These are both similar in theme, Yume Nikki involving dreams and Opus involving fiction. They both center around a vaguely mind-bending plot, although I'd hazard that Opus goes much farther with it. Their art styles are similar, plainly rendering things which shouldn't make sense.
Oh, also, the author of Opus, Satoshi Kon, made a film called Paprika. It focused on dreams, dream exploration, dream logic, etc. It was really well handled, and you can get some of its taste in Opus as well.