Through the use of isotopically pure gas at a temperature of 77 K, a weak photoabsorption band of O-16(2) is found near 1856 Angstrom, underlying the stronger Schumann-Runge (SR) band B (3)Sigma(u)(-)<--X (3)Sigma(g)(-)(8,0). The location, structure, and intensity of this new band are consistent with expectation for the magnetic dipole transition B-' (3)Pi(g)<--X (3)Sigma(g)(-)(0,0), where the designation B-' is chosen to represent the parallel to (3)Pi(g) valence state. This electronic transition contributes to the "excess absorption" underlying the SR bands [B. R. Lewis, S. T. Gibson, and E. H. Roberts, J. Chem. Phys. 115, 245 (2001)].