In japanese, expression showing the full satisfaction obtained by eating, drinking or tasting anyway, somethings really delicious (delicious is what oishii properly means), mostly, but not necessarily, related to food or drink.
Coming from the Imperial Family and the noble class of the Heian period (794-1185 AC), it's still mainly used among highly educated people and academic circles, as a way of very polite and formal appreciation, sometimes just in order to mark the intellectual superiority from the common way of speaking.
My gf...oishii dayo !!!!