Yeah, like the others said:
You can say 'OK', because we use it.
You can also use:
'Tudo bem' - it means 'everything is ok' (formal or informal form)
'Tudo certo' - it means 'everything is ok' (informal)
'Beleza' - it is a informal word, and some people don't use this, but evething get it
'Entendi' - it means 'I got it' (formal or informal)
'Certo' - It means 'right' (informal)
But all those options is for answer meaning that you got what the other person says.
If you want say you are ok, you must say:
'Eu estou bem' or 'Eu estou OK'.