We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1 - x in high-pressure synthesis. Under H2O and H-2 atmospheres, hydrogens derived from H2O or H-2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO1-xHx was obtained. However, when the H2O and H-2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calculations indicated that substitution of hydrogen in the form of H- is more stable than the formation of an oxygen vacancy at the oxygen site of SmFeAsO. These results strongly imply that oxygen-deficient SmFeAsO1-x reported previously is SmFeAsO1-xHx with hydride ion incorporated unintentionally during high-pressure synthesis.