The threshold photoelectron (TPE) spectrum of isotopically substituted molecular oxygen (O-18(2)) has been measured in the region 20.2-21.6 eV in order to determine the origin of the new progression recently observed by Baltzer et al. (Phys. Rev. A 1992, 45, 4374) by He II photoelectron spectroscopy and by Merkt et al. (Chem. Phys. 1993, 137, 479) by threshold photoelectron spectroscopy. The adiabatic ionization potential for this progression is determined to be 20.352 +/- 0.002 eV for O-16(2). Intensity perturbations in the spectrum of the B (2) Sigma(g)(-) <-- X (3) Sigma(g)(-) are also reported.