O-goshi is the first of the hip throws taught. Responding to a punch, you block, turn 180° and with bent knees, put your free arm under the armpit of the attacker and punch the floor with it, pulling the attacker round your hip in the process. For much taller or much smaller ukes, ko-tsuri-komi-goshi and o-tsuri-komi-goshi are more appropriate.
O-goshi is also useful against grabs, since you start so close to the attacker. It forms the starting point for other throws, such as harai-goshi.