In English, "okay" is pronounced /oʊˈkeɪ/ . When I hear a French saying that word, I can't hear neither the final /ɪ/ phonem, nor the /ʊ/ phonem. It seems to me /o'ke/ is said instead. Is that the most usual pronunciation?
You are right, the 1st syllable is pronounced either [o] or [ɔ], and the 2nd syllable is pronounced [e] or [ɛ] in French. Whether one is used or the other are simple regional and/or individual variations. You can hear some sound samples here.
If you use the English prononciation in a conversation in French, it will be understood, but will give away that you are a foreigner (or, if you are a native speaker, that you are a snob who wants to show off their English or American accent).