O

Obligatory wells

Exploration drillings that an operator is bound to drill in a certain area as a condition for carrying out exploration in the area (can also refer to a completely different area). 
 

OCS

Outer Continental Shelf. 
 

Oil gasification

The process by which gas is produced from oil and the gas used as fuel. 
 

Oil in place

The amounts of oil and gas presumed in an oil reservoir. 
 

Oil slick

Pool of oil (floating on the surface of the sea). 
 

Onstream

A term describing that a process installation is ready for starting production from it. 
 

Open hole (OH)

The part of the well where there is no casing. Also an operation where drilling is done without casing. 
 

Operator

A person or company that has obtained permission to drill wells and start production. 
 

Option

The right to achieve certain rights within a certain time frame; for instance on extraction rights to a field. 
 

OPEC

Organization of Petroleum Exporting Countries. 
 

Original oil or gas in place

The total amount of oil or gas in a reservoir. 
 

Outside/outer diameter (OD)

Usually casing and tubing are described in terms of inside diameter (ID) and outside diameter (OD). 
 

Outstep well

A well drilled outside of the proven oil or gas area. The well is drilled to see if the reservoir is larger than estimated. Also called step out well. 
 

Overshot

A special tool used to fish a lost piece of tubing or casing from the well.

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