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Coloration of Ophiogomphus severus is similar to its close relatives, except that O. severus has a larger proportion of black across the entire body. The head and eyes are yellow with a black line across the frons and a black vertex. Eyes do not meet dorsally. Thorax is more greenish yellow than the head and abdomen with a diagnostic dark brown oval on the prothorax. Both the forewings and the hindwings have a yellow costal vein, but dark venation color throughout the rest of the wing. Leg segments show a mottled color pattern, the leg base yellow, the tarsi black and the femur and tibia displaying both colours. Abdomen is primarily yellow and black. Lateral stripes are continuous from the first to the terminal abdominal segment while the dorsal stripe is arranged as a row of large yellow spots ending on the ninth segment. Caudal appendages are also entirely yellow. Males and females of this species look very similar, but can be differentiated by more color on the legs of the female (Needham, et al 2000).

This species is divided into two subspecies: Ophiogomphus severus severus Hagen (described above) and Ophiogomphus severus montanus Selys which is described as having a larger lateral brown spot on the prothoracic segment and more black on segment 10 of the abdomen (Needham, et. al. 2000).

In 2004, K. Gibbs et al studied six species of Ophiogomphus that were found coexisting in Maine. Ophiogomphus severus was not included in this study, but it can be assumed that it has a similar life history to its closely relatives because of their synchronous emergence patterns. Sexually mature adults are completely terrestrial and are capable of flying great distances over land in search for food, but most of the adult life is spent near water where mate selection, reproduction and oviposition take place. Mating usually occurs during June, July and August. Females oviposit their eggs close to a shallow freshwater source. Aquatic nymphs emerge shortly after and enter a series of 9 to 15 molts before reaching the final instar larval phase. Instars then overwinter in diapause, emerging from the water and undergoing complete metamorphosis two years later as a winged adult.