Time between flights at Athens Airport

Is the time sufficient to board the Olympic Air.

If the flight is not late, for sure. ATH is not big, you will be at check in by 17:00-17:30 and your check in closes 30 minutes before the flight (https://www.olympicair.com/en/Info/Checkin/check-in-times) so you have an hour buffer.

I have two different tickets. Kindly advise

Well, since you are asking for my advice. I am doing unprotected connections quite often and I always book them in such a fashion that if I am on the next plane (in this case, next day) I still can catch my connecting flight. I check into a hotel near (or, if there is one, then inside -- yay for yotel) the airport. Peeking at a site like https://www.airportia.com/flights/ek210/newark/athens/ you can see it is usually on time or less than 30 minutes late ... except July 17 when it happened to be more than three hours late. If your plane is very late, to make things worse, since I presume you are not well experienced in these matters you are very likely to spend the nine hour flight fretting over what happens: nothing much, you buy a new plane ticket. Which can be quite expensive, of course. Now, most Emirates planes have wifi so if you are very late once airborne begin to look for a new ticket. (But then again, never fully rely on your plane having wifi.)