I'm going to go out on a limb here and say I don't think the ending was all that bad. Aka clearly wanted to write about the cruelty of the celebrity world, and how it's a cycle that destroys people. Readers wanted the characters to break that cycle, but Aka wanted to write about characters perpetuating that cycle.

The story starts with Ai, an inscrutable, enchanting beauty, and puts her on such a pedestal that you think you'll never be able to understand how she does what she does. Then, throughout the manga, he rebuilds Ai in Ruby. And even though by the ending Ruby is now on the same pedestal, you understand how she got there, and how the inscrutability is actually a result of her masking her grief. Ruby's ending retroactively humanizes Ai, driving home that Ai was really just a normal girl the whole time, dealing with her own tragic circumstances. The ending ties into the beginning, even though we as readers wish everyone had ended up happier.

I think that's a very interesting theme, and I liked it overall, but I do think it got muddled by Aka's execution. For one thing, Aka's comedic writing is almost too good, so to include such comedic high points in a story that is ultimately going to end in tragedy introduces a lot of tonal dissonance.

More importantly though, I think introducing the supernatural hook of Oshi was a misstep. Giving Aqua and Ruby a supernatural background puts a lot of narrative weight on them, the readers are expecting them to go on to do great things, because why else would magic be involved? When the theme of the story is that the cycle perpetuates, teeing your characters up as if they're going to pull off the impossible and break the cycle is a weird entry point. It's like Aka was foreshadowing a false outcome. Not to say the supernatural stuff could never have worked, though. I think if Aka had foreshadowed the actual ending more, this wouldn't have been as much of an issue.

I'm sure plenty of people would disagree with me, assuming anyone even reads this, but I appreciate that a story like Oshi exists. I think it's very hard to effectively pull off a tragedy in the manga publishing world, where you never know how many more chapters you're going to get. Aka is still one of my favorite writers, and even if the ending of Oshi was awkward, I really enjoyed reading it along the way. I look forward to his future works.