ものをBut, Although, If only, I wish I had

ものを and ものの are similar in that both can express contrast (i.e. 'although'), but ものを specifically contrasts something that was expected to happen with what actually happened because the action was not done, e.g. みんなで協力すればいいものを ('although it would have been good if you had all cooperated'), while ものの can be used for general contrasts 毎朝走っているものの、足はあまり速くない ('Although I run every morning, I'm not that fast.'). Additionally, ものを will most often come at the end of a sentence, while ものの will be followed by the reason or unexpected result.

These expressions seem similar in that both use a noun followed by を to express that something is 'operating' on a given situation. ところを is often used to express that acknowledge a situation and acknowledge/describe something affecting it, e.g. お忙しいところをすみません ('I'm sorry for interrupting you at a busy time.') while ものを is often used to expect contrary-to-fact wishes about things that should have happened, but didn't, e.g. みんなで協力してやればいいものを ('If only you had cooperated and done it together, but…')

ものを and のに both are used to express the sense of 'if only'. ものを is more formal and also typically used when the speaker is expressing their disappointment at something that should have happened, not happening, e.g. 事前に知らせてくれればいいものを ('if only you had informed me in advance'), while のに can be used for general wishes 晴れればいいのに ('if only the weather were nice').