Actually, it's oissu but also ossu and uissu.
It's a contraction of ohayou gozaimasu.
And yes, you just use it with friends.
It's the same with chissu or chiisu which is a contraction of konnichiwa which again you just use with friends.
For simplicity, all of them should just be translated to what's up or 'sup. When you are greeted that, you just reply it back.