Obligatory anaerobes cannot withstand oxygen because of absence -

Obligate anaerobes may even die on exposure to oxygen. The reason for the apparent toxicity of oxygen for anaerobic bacteria is not well understood. It has been suggested that in the presence of oxygen, hydrogen peroxide and other toxic peroxides accumulate . The enzyme catalase which splits hydrogen peroxide is present in most of the aerobic bacteria but which is absent in anaerobes Ref Ananthanarayan & paniker's Textbook of Microbiology 9th edition pg no:24