A low-alloy steel for oil well pipe is provided where high strengths and good SSC resistances can be achieved in a stable manner. A low-alloy steel for oil well pipe has a chemical composition including, by mass percent, C: more than 0.45 and up to 0.65 %; Si: 0.05 to 0.50 %; Mn: 0.10 to 1.00 %; P: up to 0.020 %; S: up to 0.0020 %; Cu: up to 0.1 %; Cr: 0.40 to 1.50 %; Ni: up to 0.1 %; Mo: 0.50 to 2.50 %; Ti: up to 0.01 %; V: 0.05 to 0.25 %; Nb: 0.005 to 0.20 %; Al: 0.010 to 0.100 %; B: up to 0.0005 %; Ca: 0 to 0.003 %; O: up to 0.01 %; N: up to 0.007 %; and other elements, the steel having a microstructure consisting of tempered martensite and retained austenite in less than 2 % in volume fraction, the crystal grain size number being 9.0 or larger, the number density of carbonitride-based inclusions with a grain diameter of 50 µm or larger being 10 inclusions/100 mm 2 or smaller, and the yield strength being 965 MPa or higher.