"Ohayou Gozaimasu" is a greeting phrase which means "Good Morning". It can also be shortened to "Ohayou!" but such may be used only for informal acquaintances.

"Konnichiwa" means "Good Afternoon" but it is also commonly used like "Hi!". That's probably because afternoon is longest time of the day

Lastly, saying "Good Evening" in Nihongo is "Konbanwa"