Time (1)

When it's "on the hour" we say "o'clock". But only when it's on the hour.

12 o'clock Twelve o'clock 12 o'clock Six o'clock
12 o'clock One o'clock 12 o'clock Seven o'clock
12 o'clock Two o'clock 12 o'clock Eight o'clock
12 o'clock Three o'clock 12 o'clock Nine o'clock
12 o'clock Four o'clock 12 o'clock Ten o'clock
12 o'clock Five o'clock 12 o'clock Eleven o'clock
Because it can be difficult to say whether 12 o'clock is during the day or the night, we use two special terms.
12 o'clock 00:00 and 24:00 = Twelve midnight (or midnight)
12 o'clock 12:00 = Twelve noon (or noon)

Time (2)

In five minute increments, when it's past the hour (up to 30 minutes past) we say "past".

When it's before the hour (after 30 minutes past) we say "to".

There are 60 minutes in an hour.

30 minutes is half an hour, we say "half past" or "thirty".

15 minutes is quarter of an hour, we say "quarter past" or "fifteen" or "quarter to" or "forty-five".

Clock past to
12 o'clock Twelve o'clock 12 o'clock Twelve fifteen

or

Quarter past twelve
12 o'clock

Twelve thirty

or

Half past twelve

We never say "half to".

12 o'clock Twelve forty-five

or

Quarter to one