It is an opera.

I'm an opera singer.

to operate at full capacity

to operate a business

to operate in favour of a project

No operator may be both judge and interested party

in my opinion

Opinions differ here

Opportunity makes the thief

No opportunity offered itself

Opportunity makes thieves

opportunity for new insights

He opposed my plan, but at last I managed to bring him round

I am opposed to it.

Opposites attract

to be oppressed by the State

Be optimistic!

optional equipment

optional

or

or so

orange haze

orange

It's orange.

orange juice