As others have previously stated, o'clock is used for exact hours but I would also add that you can practice saying "6 minutes to 7 o'clock" if you want to incorporate o'clock into your sentence structure.  

No, you can't say that. We can only use o'clock for times which are precisely 'on the hour':

It's at 6 o'clock

It's at 6.54 = It's at six minutes to seven

It's at 4.50 =  It's at ten to five (we don't need to say 'minutes' for minutes which are multiples of five)

As previously said, o´clock is for precise on the hour

However, there is another expression you can use, if the meaning you want to convey is a exact time, for example

The concert would start at 8:45 sharp

which mean the concert will star at 8:45, not at 8:44 or 8.46

Yo C,

Are you talking about what time an event starts?  Or are you talking about what time it is right now?  Your times that you give as examples make me think that you are talking about saying what time it is.

What time does the concert start?     It starts at 6 p.m.  (Or, it starts at 6 o'clock.)

What time is it?  

It's 6 o'clock.

It's 7:53.  (seven-fifty-three)

It's a quarter to 6.

It's half past 7.

It's 5 to 7.

It's a quarter past 3.

Many young people do not understand the phrases quarter past, quarter to, or half past because they can not read a regular clock.