DOM is actually not a type of tubing, but a process that is applied to tubing after it is initially constructed. It is Drawn Over a Mandrel...which "cold works" it, giving it more exact dimensions relative to the inside and outside diameters, a smoother finish, and better alignment of the crystal lattice structure. Although it is almost always referred to as a SEAMLESS tube, technically it is NOT seamless tubing, and it started life as some sort of EW (electric welded) tubing. During the manufacturing process, the weld line becomes nearly undetectable, thus it is referred to as SEAMLESS. It is considered a high strength, high quality tube, and is normally constructed from SAE 1020 or 1026 steel. DOM is commonly used in the manufacturing of race cars and motorcycle frames.
1020 is normally used in the manufacturing of small-diameter or thin-wall DOM steel tube. 1026 grade is normally used in the manufacturing of DOM over 2" OD, with walls heavier than .156". Depending on the diameter and/or the wall thickness, you may receive EITHER 1020 or 1026.