WISE TO THE LIES.

storm && onyx.

honestly, i think they'd be a great match for each other! he would adore that she's so driven and hard working and would take great pride in her ability to learn fast. onyx has very high expectations of his apprentices and i think her sharp tongue and stubborn nature would do good to humanize him a little (even though he's a cat? hahaha). sometimes he's too much work and no play, so her ability to crack that hard shell of his would do wonders. they could be very close! her obvious respect for authority would appeal to him as he only dishes out what he is given, so he'd very much respect her desire to succeed within the clan.

red && onyx.

and i love the little deputy rivalry! onyx might not outright say it, but he's incredibly bias toward his own clan. he likes to think only the best of the best resides in shadowclan and the others come second. he's a bit of a jerk that way. but that's what gives him the potential to be incredibly lethal, because all that matters is shadowclan's survival, any he'll smear anything off the map that stands in the way of that. onyx would have a slight degree of respect for red, because he'd recognize that the other deputy would have to be loyal and committed to attain that position. at the same time, he'd find it discomforting, as anyone outside of shadowclan becomes a natural enemy to him.

Cinders.

i think that idea is brilliant! onyxfeather would always be in denial, he'd probably see love as a weakness, something to hold him back, and that's why they won't be together. he spends too much time on clan affairs than working on his own relationships, maybe that's why they fight? i could see him being very off-putish, as he's always struggled with affection and emotion, so he'd try to deny himself what feelings he might have for her. but he'd always try an push her away, play hot and cold like he wasn't interested anymore, but get defensive and nosy when she's around other toms. it could be quite comical actually! i like your idea very much!

EDIT.
secretly, he's terrified of kits, and love. if the workaholic thing wasn't enough to drive them apart, he'd make excuses. he'd probably be super overwhelmed if things had ever gotten serious and high tail it out of there. onyxfeather is terrified of being a father and doesn't really no how to cope with affection. so that might also contribute!

yay! i'm actually so excited! hahaha. /squeal!

puddlesdog

dawn && onyx.
i like that a lot! it'd definitely throw a twist in with skyfeather. i think onyxfeather might take a bit of an interest in her, or at least do so just to spite skyfeather and make her jealous. he would be sympathetic to dawn, having lost her previous lover.

redfeather && onyx.
onyxfeather definitely has the potential to be very lethal, especially toward the other clans. he would stop at nothing to make sure shadowclan succeeds, even if that means attacking the other clans. if redfeather saw this trait in him, it wouldn't be hard for the medicine cat to be uneasy with him, or dislike him!