In oil drilling, a casing head is a simple metal flange

Flange

A flange is an external or internal ridge, or rim , for strength, as the flange of an iron beam such as an I-beam or a T-beam; or for attachment to another object, as the flange on the end of a pipe, steam cylinder, etc., or on the lens mount of a camera; or for a flange of a rail car or tram wheel...

 welded or screwed on to the top of the conductor pipe

Conductor pipe

The Conductor Pipe is a large diameter pile that is set into the ground to provide the initial stable structural foundation for a borehole or oil well....

 (also known as drive-pipe) or the casing and forms part of the wellhead

Wellhead

A wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....

 system for the well.

It is the primary interface for the surface pressure control equipment, such as the blowout preventer

Blowout preventer

A blowout preventer is a large, specialized valve used to seal, control and monitor oil and gas wells. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic...

s (for well drilling) or the Christmas tree

Christmas tree (oil well)

In petroleum and natural gas extraction, a Christmas tree, or "tree", is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells...

 (for well production).

The casing head, when installed, is typically tested to very strict pressure and leak-off parameters to insure viability under blowout

Blowout (well drilling)

A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....

conditions, before any surface equipment is installed.