What is the difference between '8:00 o'clock' and saying only '8' in writing letters?
When we can use the first one and when we can use the second one?
Unless it is overly formal, I would leave off the o'clock. And you should add AM or PM :
I am planning to go to the beach at 8AM on Friday.
But if it is very clear in context, you could just use the numeral:
After I left work, I met Laura at 8.
(AmE applicable to the 12 hour clock system.)