Are these sentences OK?
Come at ten sharp. / Come at ten o'clock sharp.
Come over at ten sharp. / Come over at ten o'clock sharp.
I remember how cold it gets there. / I remember how hot it gets there.
He always smelt of cigarettes. / He always smelt of alcohol.

Question

Are these sentences OK?
Come at ten sharp. / Come at ten o'clock sharp. 
Come over at ten sharp. / Come over at ten o'clock sharp.
I remember how cold it gets there. / I remember how hot it gets there.
He always smelt of cigarettes.  / He always smelt of alcohol.

Answer

In speech these sentences are absolutely fine.

I would probably include a comma;

Come at ten, sharp!

Etc