Rotational spectra of OH and its isotopologues have been precisely measured using high efficiency terahertz (THz) sources. The measurements are compared with existing data and are useful for global modeling. For the first time, microwave measurements of the Lambda-doubling transitions of the (OH)-O-17 isotopologue are combined with THz data successfully. Precise rotational, fine-structure, and hyperfine structure parameters for the (OH)-O-17 isotopologue are reported. An isotopically independent Dunham model for all isotopologues of (2)Pi OH nu < 3 is presented.