Olam has a diverse set of meanings, including, world, existence, lifetime and eternity. As is often the case in Hebrew, the etymology of the word suggests the link between its different senses; olam comes from alam, a verb meaning to hide or conceal. The underlying theological idea is that whether in time or in space, the olam is that which conceals the presence of God in the world.
Olam haba is the world to come. Considering the diverse and undetermined nature of Jewish teaching about olam haba, it is a remarkably common phrase. Whether olam haba is a separately existing place, or a dimension of existence somehow concurrent with this world, as Maimonides says, or an individual creation of our own good and bad acts as Rabbi Chaim of Volozhin seems to suggest, or a place or reward and punishment, as the Talmud says, or the arena of reincarnation, as some Kabbalists teach, are topics of hot dispute.
In Yiddish, the oilam (or sometimes the ganze oilam) refers to the known world, or the community of people like you. One could say that the oilam wears felt hats, or that the ganze oilam knows one shouldn’t believe such and such gossip.