Phrasal Verbs are idiomatic expressions, combining verbs and prepositions to make new verbs whose meaning is often not obvious from the dictionary definitions of the individual words. They are widely used in both written and spoken English, and new ones are formed all the time as they are a flexible way of creating new terms.
A reference page of 2,570 current English Phrasal Verbs (also called multi-word verbs) with definitions and examples is here.
Phrasal Verbs beginning with O :
Occur to : Enter one's mind
Didn't it OCCUR TO you to help me when you saw how much trouble I was in?
Open up :
1. Start to talk freely about something
She hates to OPEN UP and discuss her feelings.
2. Open a shop or business for the day
They OPEN UP at seven in the morning.
3. Allow goods into a market
Before they joined the WTO, they had to OPEN UP their markets.
Opt for : Choose
I OPTED FOR an endowment mortgage and lost a lot of money.
Opt out : Choose not to be part of something
The UK OPTED OUT of a lot of EU legislation on working hours and conditions.
Owe to : Be the reason for something
She OWES her success TO hard work and determination.