Answering about the time

Question

1. I get up at 6 o'clock in the morning.
2. I get up at 6 in the morning.

Can I use sentense 2? When can I use only number without o'clock am pm etc?

Answer 1

Really, you only have to say o'clock if there is any chance the listener/reader might think you are not talking about a time, or if you are in a very formal communication setting.

These situations are not super common in conversation at least, so usually you will see or hear o'clock only if someone is telling you things like the rules or guidelines, or in warnings or advertisements where they want to be 100% clear.

One situation where it's tended to be used is if you are responding to a question with a time.

A: What time do you get up in the morning?

B: 6.

This is fine, but if you want to make it more clear, i.e. you think A is not listening closely, responding "6 o'clock" would be the tendency.

Answer 2

Your two sentences are interchangeable and have the same meaning.

When an absolute reference to times given, usually "o'clock" is assumed.

We had lunch at 1:00.
we had lunch t 1 o'clock.

He got home at 6:00. he got home at 6 o'clock

AM and PM designations are not always necessary since additional/assumed context will provide it. To ensure time is unambiguous, you could use military time or 24-hour clock where PM hours are added to "12" to create a 24-hour clock. However, most people do not communicate this way in everyday usage.

We had lunch at 1:00.
we had lunch at 13 hundred hours.

He woke up at 6AM.
he woke up at O-six hundred hours.