We say olha at the beginning of sentences to get someone's attention and sometimes you will hear when people give advices •Getting someone's attention. - Olha, eu achei uma nota de 2 reais na rua. Look, I found a 2 reais bill on the street. -Olha o tanto de presentes que eu ganhei de aniversário. Look how many gifts I got for my birthday. •Giving advices -Olha, eu acho que você não deveria matar aula. Look, I think you shouldn't skip class. Olha, eu não acho que você deva gastar tanto dinheiro assim. Look, I don't think you should spend so much money like that. *We use "olha" the same way you guys use "look". There's no secret.

"Olha" means like you said, "look" but we also use it a lot in the beginning of the sentences in order to call atention from the listener.

Hey, Alex. In fact brazilians use not only "olha", depends what city you are from in Brazil. For example, I'm from a small city near São Paulo and we use "Viu". We don't use with an especificly mean, it's like "Hey" for you when you want the attention of the other person.