ものを

1. 1Laments that a simpler or better course of action was available, yet an undesirable outcome happened instead.Ex. 1
2. 2Criticizes someone for not taking an obvious, easy option (such as staying quiet) and causing trouble as a result.Ex. 2

A somewhat literary pattern used in writing and in reflective or critical speech to lament a missed chance or criticize someone's choice. The part before ものを presents what would have been better. What follows either states the regrettable reality or is omitted, leaving the reproach hanging. It resembles のに but sounds more emotional and bookish, with a stronger sense of "it would have been so easy."

• のにN4「のに」 likewise expresses concession and dissatisfaction and can also end a sentence, but its register is neutral, it is common in speech, and it can express mere surprise. A 「ものを」 sentence can generally be rewritten with 「のに」, but a mild 「のに」 sentence should not be rewritten with 「ものを」.
• ばよかったN3「ばよかった」 is by itself a complete expression of regret, typically ending the sentence or followed by 「のに」, with a neutral register; 「ものを」 is a literary concessive that can follow it (「ばよかったものを」) to introduce an undesirable outcome, with a stronger tone of reproach.
• くせにN3「くせに」 is colloquial, carries stronger blame often with contempt, and criticizes someone whose words and actions do not match their status or ability; it takes the same subject in both clauses and can attach to 「名詞＋の」. It resembles 「ものを」 mainly in uses involving established facts, and it can also attach to 「〜ばよかった」.
• ことかN3「ことか」 pairs with words like 「どんなに」 and 「何度」 to exclaim about how great something is or how many times it happens, and does not express concession; 「ものを」 expresses the regret and dissatisfaction of 'would have... but...', and its sentence-final use also omits the following clause. They are not interchangeable.
• とはN1「とは」 expresses surprise, deep emotion, or indignation at something unexpected, and does not express concession; 「ものを」 is a concessive carrying regret or reproach. Both can end the sentence with the following clause omitted, but one conveys astonishment while the other conveys regret and dissatisfaction, so they are not interchangeable.

のに is the everyday equivalent and lacks ものを's literary tone and emphasis on an easy option that was missed. ばよかった simply states regret, whereas ものを adds a reproachful contrast with what actually happened, stated or implied. くせに attacks a person's inconsistent behavior, while ことか (exclamation of degree) and とは (surprise) express no regret over a missed option.