The spectrum associated with the J = 3 <-- 2 transition of OH+ in the a (1) Delta(v = 0) state has been observed by far infrared laser magnetic resonance spectroscopy. A new microwave discharge source enabled the detection of this spectrum, which is the first observation of the rotational spectrum of an ion in a metastable state. Assignment and least-squares fitting of the observed transitions have determined the following molecular constants : B-0 = 494.420 388 (22) GHz, the proton hyperfine parameter a = 74.84 (32) MHz, g(L)’ = 1.000 915 (15), and g(r) = -0.001 815 (18), with the 1 sigma uncertainties of the last digits in parentheses. The relationship of these parameters to the geometric and electronic structure of OH+ is discussed.