According to The Online Etymology Dictionary, the origin of the world "Oracle" c.1384, "...is a message from a god, expressed by divine inspiration," from O.Fr. oracle (12c.), from L. oraculum "divine announcement, oracle," from orare "pray, plead" (see orator), with material instrumental suffix -culo-. In antiquity, "the agency or medium of a god," also "the place where such divine utterances were given." This sense is attested in Eng. from c.1400."
It is often used to describe something or someone who can divine or predict or see into the past or the future. I prefer to think of it as something that opens us up to new potentials for learning, a tool that opens doors. In the smallest sense, it is permission to change long held and cherished beliefs that need some fresh air. If "The Oracle" says it is true, then surely it is okay to open the door.
I've chosen these links for both the familiarity of their names, such as Facade, Llewellyn, and others who made simple esoteric knowledge available to anyone within reach of a computer, all the way to strange and unique applications and reworkings of those familiar systems.
I've also included several horoscope sites, both traditional and weird, eastern and western, new age and old age as these are some of the oldest and most familiar oracles. For many people, the daily horoscope is as much a part of their morning as the cup of tea or coffee.
I make no claim to the validity of any of these. In the end it all comes down to the same thing: we are our own teachers and everything is a lesson.