Answer by Professional Japanese Teacher
We will introduce two ways to say a quick “Hi” or “Hey” in Japanese but take note that all of these are very casual and should only be used for close friends or family members. They are also used predominantly by males. For formal versions of “Hello”, click here.
You sometimes hear guys speak like this in anime. Ossu is actually a shortened version of Ohayoogozaimasu by taking the o and the su at the beginning and end of the phrase to form Ossu. Note that this is super casual, and you should not use it for someone senior. Also note that the final u is not pronounced, so this word sounds more like “oss”.
This is mostly said by guys to close friends.
Do try these greetings next time!