Students often use the expression “ça va” as an adjective. I’ve heard, for example, “Le film était ça va,” to mean the film was “OK.” A French person would have no clue what that sentence meant.
“Ça va” means “it’s OK,” in the sense that things are going well, but it is a sentence unto itself rather than an adjective.
If you need an adjective, “acceptable” would do or “médiocre.”
To incorporate “OK” in a sentence, you would need the appropriate tense of “aller” and, most likely, since it’s informal, the subject “ça” with no other verb. For example, in response to the question, “How was the film yesterday?” one could say “Ça allait,” meaning “it was OK.” If someone is worried about the future, a friend could respond reassuringly that “Ça ira” in the future.