In my opinion, it's gonna be hard to find something close to OnK. It mixes some elements that are hard to find together in other works. The closest thing I can think of is Perfect Blue.
There are some other shows that come to my mind, but they only share similarities with some aspects of OnK, not all of them.
If you liked the insight into the entertainment industry, you might like Shirobako, or Showa Genroku Rakugo Shinju if it was the performing arts that caught your attention.
If you liked the dramatic take on
OnK S1E1
reliving your life, then ReLIFE might be to your liking.
If the main thing that caught your attention were realistic human interactions, then you might find some of these interesting, even though some of them are completely different thematically:
If it was the
general OnK spoiler
revenge plot that you liked, then there's Vinland Saga and Gankutsuou, even though I personally didn't like the latter.