The Object Marker Particle "wo" (を)

The particle を (pronounced "o") is one of the most fundamental grammatical markers in Japanese. Its sole purpose is to mark the direct object of a transitive verb. For example, in the sentence "hon o yomu" (to read a book), hon (book) is the direct object and is marked by を, while yomu is the action.

Note that を is only used with transitive verbs where an action is actively done to an object. In modern Japanese, the character を is used exclusively as a particle and is pronounced exactly like お (o).