Background: The activity concentrations of some radionuclides in soil samples collected from 20 locations in 8 major towns in Oyo State, Southwestern Nigeria have been determined by gamma-ray spectrometry technique. Materials and Methods: The activity concentrations were determined using high-purity germanium (HPGe) detector (Canberra Industries Inc.). The outdoor absorbed dose rates in air at about 1.0 m height were estimated from the activity concentrations and dose rate conversion factors for the radionuclides. The annual outdoor effective dose equivalent rates were also estimated for urban and rural areas of the state using the calculated absorbed dose rates in air. Results: Determined specific activity concentrations of the radionuclides ranged from 1±0.4 Bq kg^-1 for ¹³⁷Cs to 1190±30 Bq kg^-1 for ⁴⁰K. The estimated outdoor absorbed dose rates in air varied from 52 nGy h^-1 in Egbeda (a rural area) to 414 nGy h^-1 in Eruwa (also a rural area). The mean annual outdoor effective dose equivalent for the urban areas in the state was 0.1 mSv y^-1 while that of rural areas was 0.3 mSv y^-1 with a standard deviation of 0.02 mSv y^-1 and 0.3 mSv y^-1 respectively. The mean for the study area was 0.2 mSv y^-1. Conclusion: The mean annual outdoor effective dose values for the urban and rural areas, for the whole study area are higher than the world average annual outdoor terrestrial radiation value of 0.07 mSv y^-1 reported by the United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR) for individual members of the public.