A tunable low−temperature rf oscillator with Si MOSFETs as active devices is described. Tunability by a factor of 2 in frequency is reached by using GaAsP varactor diodes. Used as an NMR probe with27Al and51V, the oscillator allows magnetic fields to be measured in superconducting magnets with an accuracy of better than 0.1 G. We found the gyromagnetic ratio &ggr; of27Al and51V at 4.2 K to be 1.11119±1 and 1.12555±1 MHz/kG, respectively, corresponding to a Knight shift of + 1.61×10−3and + 5.58×10−3, respectively.