A Hall probe was inserted into the magnet of an optically detected magnetic resonance (ODMR) spectrometer to study residual field at zero current, homogeneity, and hysteretic effects. Residual fields were on the order of 10 mT at zero current. The magnet was homogeneous to within one part in 8000 or better over a region of 2.5 cm along its vertical axis. Several hysteresis loops were studied for field sweeps. The largest shifts were about 10 mT. Using the information collected, recommendations are made on how to improve precise field measurements for ODMR.