We probe the interconversion of spin permutation symmetry of weakly bound electron-hole carrier pairs in an organic light-emitting diode by monitoring the changes in yield of recombinant species-singlet and triplet excitons-through fluorescence and phosphorescence, respectively. Spin mixing occurs by spin precession in local hyperfine fields and is suppressed by an external magnetic field, leading to an anticorrelation of fluorescence and phosphorescence yield, which follows the same functionality as magnetoresistance. A resonant radio-frequency field reverses this effect, enhancing spin mixing to raise the phosphorescence and lower the fluorescence. The experiment offers a direct simultaneous optical probe of the two interconverting spin states in the radical-pair mechanism, which features prominently in models of biological magnetoception.