X-band electron spin resonance (ESR) spectroscopy has been applied to the study of molecular rotation of O-2 molecules in isotopic solid hydrogen, D-2 and HD. ESR signal of the O-2 molecules in hindered rotational states has been observed, and its pressure dependence has been measured up to 19 MPa. Although molar volume of solid hydrogen decreases, the O-2 rotation has become less hindered with the increase in pressure and isotope substitution from HD to D-2. These effects of pressure and isotope substitution suggest that the potential barrier for the O-2 rotation is mainly produced by distortion of O-2 cages in D-2 and HD. Since solid hydrogen becomes less compressible with the increase in pressure and the isotope substitution from HD to D-2, the O-2 rotation becomes less hindered in the less distorted O-2 cages.