If you have been introduced to any worker, not just the CEO, it's perfectly fine to say hi, and would be considered rude in some circumstances not to. But what matters greatly is the context, and indeed the manner in-which you communicate.
You can't walk into a meeting, look across the room and greet the CEO.
You also should refrain from seeking them out, just to say hi. They will be busy.
Also, keep in mind, that brown-nosers are spotted a mile away. Don't be that guy.
If you bump into them in the hallway, you should certainly feel free to say hi. If they are speaking with a client or customer, or with people you are not familiar with, you should probably refrain from making the first move. It's better just to do a small nod if you both make eye contact.
Also, senior management, and even interviewers often meet hundreds of people a year, and it can be hard to keep track of them all. So don't worry if they don't remember you.
You will get a feel for the workplace culture at your place. At the places I've worked at for example, the CEOs will happily sit down for lunch with random workers (after being invited) and have a chat over lunch. (And these are companies with over 1000 workers). But equally, there may be places where the CEOs never do this.