Ockeghem is astonishing. First, for the constant invention of his tunes. Ockeghem makes all the familiar moves, locally, but assembles the moves into long and unpredictable continuities (someone, certainly, is at this moment writing a dissertation on the Fractal Complexity of Ockeghem's Melodic Lines). Second, for shocking moments — like that above — is which the diversity of lines in the ensemble converge — here going absolutely in your face with a cross-relation f#-f'-natural, enjoying a direct move from one extreme to the other of his tonal resources* — into something approaching, when not actually, epiphanic.
*Come to think of it, a lot of modern music gets by with the shock value of shifts no greater than O.'s here, from a b minor triad to a d minor triad. Heck, that's just about enough harmonic material for a very satisfying evening at the Mabou Mines. (Yes, Virginia, O. didn't think of it in those terms, but have you any doubt that he heard it?).