I would say that the only vestiges of を's old use as a conjunction (meaning something like "although") is in the conjunction ものを, which usually ends up getting used at the end of utterances in my experience. Ex. 昨日豆乳を買ってきてよかったものを。(~"If I had only bought soy milk yesterday"). It's very similar to のに but more restricted in its use. If any native speakers could weigh in on this it would be appreciated (the memory's a bit fuzzy).