@Grackie yay~ i'll add that to my list :>

http://www.asianfanfics.com/story/view/1051994/the-caught-admirer-fluff-mature-romance-exo-exok-baekhyun-baekhyunoc

Cough, cough. I have two baekhyunxoc fics, and one sehunxoc one shot. But stories that aren’t mine that I recommend? The Caught Admirer by EmilyxXD..I’ll dig up the link for you

I totally agree with Good Girl! here are some others from a quick scan of my subscriptions :)
Seven (Good but dark. Also, it is only part 1 of a series in this AU so the "harem" part is not fully defined): https://www.asianfanfics.com/story/view/1113642/seven-book-1-angst-fantasy-romance-reverseharem-exo-ot12-postapocalypticau
But what do you mean you are a girl (picture comedy and genderbender tags where it is from a gender ambiguous OC and not one of the EXO boys becoming a girl, and this is what you get. plus it is well written, so bonus.): https://www.asianfanfics.com/story/view/1036260/but-what-do-you-mean-you-re-a-girl-comedy-crackfic-genderbender-harem-originalcharacter-exo
The Sons of Venus (this is the OMG Rated R kind of reverse harem, so be prepared for some pretty hardcore . also it is not OT12 harem, but instead a mix of boy groups which to me was refreshing): https://www.asianfanfics.com/story/view/990809/the-sons-of-venus-fantasy-exo-bap-got7
The Fallen (Saving the best for last. Not explicitly a reverse harem, but it gets there. the caution is how long this story is, but the chapters are not too long individually and it is decently well written so there is no difficulty reading it): https://www.asianfanfics.com/story/view/828469/the-fallen-fantasy-schoollife-originalcharacter-exo-supernaturalpower-wolfau-mama00au

Lost in Translation- by xiubaek13
Good Girl & Alpha Omega- both by prettybird