Ohayo Gozaimasu – Good morning. Standard greeting used by people when they meet for the first time in the morning at home, on the way to work or school or as the working day begins. Bar hostesses will use this to greet eachother as they come to work in the evening. You reply to it by repeating it back! In informal speech it is abbreviated to “Ohayo”.
Oppai – Breasts/Boobs. It also can mean slang for “Mother’s Milk”.
Otaku – Die-hard fan, nerd in Western World, but mostly Unhealthy Obsessive Interests in Japan. In Western Europe and the US some people call themselves, with a certain pride, “otaku” to indicate that they are anime fans. In Japan however otaku has not only a broader meaning but it is also used in a negative sense. In Japan, an otaku is a social outcast, a pariah of sorts. A loner with a certain obsession for which he sacrifices everything else. That obsession can be anime and/or manga but it can also be something else like cars, an idol singer, a photo model, an theatre performer, a sportsman, … . Take an idol singer otaku for example. He must absolutely have everything that is related to her. He must also know everything about her, even the smallest details such as to which elementary school she went when she was still a kid. Nothing else really matters. He will meet with fellow otaku on a regular basis and they will try to beat each other with the size of their collection and the extent of their knowledge. Occidental examples of otakus can be found, for example, among the Trekkies (fans of Star Trek).