Both are fine grammatically but both are fairly formal from my point of view. It's not something I would generally expect to hear in everyday conversation with a friend. It sounds more like something a character in a book might say.
I was at the cinema at 7 o'clock last night.
At 7 o'clock last night I was at the cinema.
Policeman investigating a crime:
- Where were you at 7 o'clock last night?
- Hmmm, let's see. Last night...at 7 o'clock...I was at the cinema. That's right.
Both are fully grammatical,
but I prefer the one that goes from general (yesterday evening) to specific (7 o'clock).
If you say "at 7 o'clock" first, there is (theoretically) a very brief moment when the listener doesn't know whether it's a.m. or p.m.