What's the difference between "I make it nearly 10 o' clock" &"It's nearly 10 o' clock."?
And in which sentence one is more preferred over the other?

Question

Answer

The question to the first sentence would be: What time do you make it? (= "What time do you think it is?")
The question to the second sentence would be: What time is it?

Answer

The second variant is correct.

Answer

"It's nearly 10 o'clock" (pay attention to the apostrophe please - no space!) is just a statement of fact.

"I make it nearly 10 o'clock" is giving your opinion - however, this is a fairly colloquial usage of "make", so be careful. A more standard way to express this is "I think it's nearly ten o'clock."