BackgroundDespite the large-scale investment in access to contraceptives, high population growth and unintended pregnancies are posing pressures in Ethiopia where the economy is incapable of holding overpopulation. The aim of this study was to assess and explore socioeconomic and religious differentials in contraceptive uptake.MethodsA mixed-methods phenomenological study was conducted in western Ethiopia, Oromia region. Data were collected through survey with 1352 mothers of reproductive age, interviews with 37 key informants, and 13 focus group discussions with family planning service providers, college instructors and mothers of reproductive age. Multivariate logistic regression model was used to identify factors associated with contraceptives uptake and thematic analysis was used to interpret the qualitative data.ResultsOf mothers included, 68% lived in rural settings and 50% were unable to read and write. Religiously, 42% were Protestant Christian, 30% Orthodox Christian and 25% Muslim. Modern-contraceptives were available at healthcare facilities; however, all mothers have been influenced by religion not to use contraceptives. Muslims were 65% less likely to utilize modern-contraceptives as compared to Orthodox (aOR, .35, 95% CI, .21–.60). All mothers were well informed of any one of modern-contraceptive methods and knew a place to get the service, while their knowledge about contraceptive was limited and their contraceptive uptake was low.ConclusionThough the Ethiopian government has so far improved access to contraceptives, utilization is lagging, mainly due to religious influences, limited contraceptives knowledge in the community, and low home-based contraceptive coverage. Societal attitudes and norms of the community towards modern-contraceptives need to be modified through innovative and culturally appropriate interventions. In countries like Ethiopia, where people’s religious devotion remains reasonably high, knowledge on natural-contraceptive methods is equally important to help religious people make an informed decision about family planning in accordance with their faith.Electronic supplementary materialThe online version of this article (10.1186/s12905-018-0580-6) contains supplementary material, which is available to authorized users.