Reflexive pronouns are used instead of object pronouns when the object is the same individual as the subject of the sentence. Thus "I can see myself [not "me"] in the mirror. "Oneself" is the reflexive pronoun corresponding to the third person impersonal subject pronoun "one": "One should keep oneself out of trouble". Note however that compared to other European languages (I am familiar with French and German in this respect, less so with Spanish, but I wouldn't be surprised if there are similar considerations there) that (a) "ones" is used sparingly and may be seen as rather formal ("you should keep out of trouble", not necessarily directed at your audience, may be more usual), and (b) reflexive pronouns are used less than in those languages, so it is usually "I washed" rather than "I washed myself".
The same form of pronouns can also be used non-reflexively for emphasis, thus "I myself wouldn't do that". (I'm having a hard time thinking of a convincing example there using "one"!)