올 is a future tense or tenseless adjective from of the verb 오다 (to come). It is often used to mean "coming", "upcoming", or "this" when referring to a time. It usually means an upcoming year or season, but in some cases it can also mean the period you're in now or even the season that has past (e.g. 올여름은 너무 더웠다 = It was so hot this past summer). It also functions as a prefix in some words, as in 올해(this year), 올봄(this spring), 올가을(this fall), etc. You can also say things like 올 생일 (= 금년 생일), 올 장마 (this year's rainy season). Aside from this "season" words, you can use it as a general adjective for "coming" or "to come", as in 올 사람 (people who are coming), 아무도 올 일 없는 외진 이곳 (This remote location no one would have any reason to come).

I've seen some people used "올~~~!" as the meaning of "Wow!" in messaging.

you have to make sentence with word 올 just 올 makes no sence