The Oxygen Pipe is added by Galacticraft. It is used in the creation of the Sealable Oxygen Pipe, Oxygen Gear, and the Air Lock. It is also used to pipe oxygen from an Oxygen Collector to an Oxygen Bubble Distributor or Oxygen Compressor.
Dyed Oxygen Pipe
It is possible to dye oxygen pipes just by applying dyes directly on them. Simply right-click on a pipe with the desired color of dye on your hand. One piece of dye will be used for each piece of pipe to be dyed. Since pipes of different colors will not interconnect, this method can be used to keep the pipes apart and separate supply systems when there is not enough space.
Breaking a dyed oxygen pipe will drop the pipe itself with no color and the dye.
This video is an example of dyed oxygen pipes.