"It's ok" is not clear. Depending on what they are offering you, the other person might take that to mean "Sure go ahead!".

Yes, I agree with Jason - what is 'it'?!! Perhaps: 'Oh it's OK, I'm not hungry!' or, 'It's OK - I'm about to go out for dinner!' But just it's OK when someone is offering could be confusing. Of course, adding a gentle hand gesture could help, but I would stick with 'I'm OK thanks', or 'No thank you', or 'I'm alright, thanks', would be better! :)