を used only when it's a grammatical particle denoting the direct object like in りんごを食べる お is used elsewhere

を is a particle for direct objects, and is used as just that. Maybe in historical hiragana, を may be used (such as 男, which has the historical reading of [をとこ]. Otherwise, it isn't ever used in the pronunciation of any modern Japanese word, mainly because the Japanese got to a point where the "w" in "wo" (That's the more accurate romanization of the kana) was not able to be pronounced. 私はテレビを見て居ます。[わたしはテレビを見ています。] translates into "I am watching TV." を is the direct object marker, proceeding テレビ, thus defining what the action (見て居ます) was directed towards, which was the television. It can also be used to indicate the moving out of a place (contrast with に), or the moving through a place. Example: 公園を出ます。（こえんをでます。）means "(I) exited out of the park", or alternatively "(I) left the park." 公園を歩きました。（こうえんをあるきました。）means, "I walked through the park." On the other hand, "o" or "go" (お/ご, respectively), is used as an honorific for words. There are words that are embellished when the prefix is used (ex. おすし), and then there are words that NEED to be embellished (ex. おもちゃ, which means toy). I believe these are also the words that CAN go unembellished, but would otherwise make you sound ill-bred, rude, or even a drunkard (お金 「おかね」 is ALWAYS usually said as just that, not just 「金｣). Then, you have words whose meanings CHANGE when the prefix is used (ex. [お中] means "stomach", while [中] by itself means "middle" or "center".）を is ALWAYS used as a particle, and therefore it's katakana equivalent ヲ is seldom used. Except for certain instances, お and ご are optional and primarily embellish words that you sound more polite/formal/humble. Those two aforementioned kana will NEVER be used as particles.