Radiotherapy of pelvic cancers may cause severe tissue injuries, and hyperbaric oxygen therapy (HBOT) is one of few treatment alternatives. As part of a longitudinal, mixed-methods study, this study’s aim was to explore pelvic cancer survivors’ experiences of undergoing such treatment. Using a phenomenological-hermeneutical design, in-depth interviews of 20 cancer survivors were conducted and analysed using systematic text condensation. This study is reported in accordance with COREQ. The informants’ experiences were identified as: Approaching an unknown world, From feeling worried to becoming familiar, A long-lasting treatment course, and The treatment course went better than expected. Despite information prior to the treatment, informants were worried about HBOT but were still motivated to try it. A combination of relevant information, clear routines, person-centred care, peer support, and limited side effects seem to be important factors for patients’ experiences of safety from this treatment.