OCTG divided into oil casing and oil tubing.

Casing is a large-diameter pipe that serves as the structural retainer for the walls of oil and gas wells, or wellbore. It is inserted into a well bore and cemented in place to protect both subsurface formations and the wellbore from collapsing and to allow drilling fluid to circulate and extraction to take place.

The outer diameter generally is 4-1/2"to 13-3/8"

Tubing is pipe used for the transportation of crude oil and natural gas from an oil or gas layer to the surface after drilling is complete. It is made to withstand the pressure generated from the extraction process. Tubing is manufactured in the same way as casing, except that an additional process known as "upsetting" is applied to thicken the pipes.

The outer diameter generally is 2-3/8" to 4-1/2".