Getting an orgasm while traveling. Generally one person is driving while reaching over to finger fuck the other, but any act of O-ing while traveling would be considered an O on the Go.
Its a brazilian slang used by guys when they just broke up and are single or when they´re having a lot of fun being single. Or it can be used when a guy is down for everything. You can use "o pai tá on" to flirt too.
Neymar used this phrase to celebrate the classification of Paris Saint-Germain (PSG) for the Champions League semifinals.