We usually say OK, but you can use "Tudo bem" to be more formal, "Tá", "Beleza", "Certo" to be more informal.

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'.

I've heard some say "Certo" or "Tá bom". It depends on the context of the sentence though. If you just want to agree with someone, I would say "Tá bom" whereas if you want to say something like: That answer is okay, then you would want to say something like - Essa resposta está certa. Hope this helps!