FRANK OCEANSues ProducerYOU DIDN'T WRITE 'BLONDE'

Frank Ocean's in a legal war with a guy he used to work with, but who's now trying to take credit for Frank's music ... according to Frank.

Frank's suing Om'Mas Keith, whom Frank says he hired in 2014 to produce master recordings for his "Blonde" album. Problem is ... Frank says Om'Mas is now trying to get credit for co-writing a bunch of those songs.

In docs, obtained by TMZ, Frank says Om'Mas registered with ASCAP -- which manages royalties for songwriters -- and claimed he had co-written 11 songs ... including "Pink + White," and "Ivy."

Frank says he hired and paid Om'Mas a flat fee to work on the master recordings ... and that's it. Frank's adamant -- Om'Mas didn't write the lyrics, melody or music. He's suing to get a judge to block Om'Mas from getting songwriting credits.