Using a tunable, narrow-bandwidth vacuum-ultraviolet source based on third-harmonic generation from excimer-pumped dye-laser radiation, the F (3)Pi(u)<--X (3)Sigma(g)(-)(1,0) photoabsorption cross sections of O-16(2) and O-18(2) have been recorded in high resolution. Rotational analyses have been performed and the resultant F(v=1) term values fitted to the (3)Pi Hamiltonian of Brown and Merer [J. Mol. Spectrosc. 74, 488 (1979)]. A large rotationless isotope effect is observed in the F(v=1) predissociation, wherein the Lorentzian linewidth component for O-18(2) is a factor of similar to50 smaller than the corresponding O-16(2) linewidth. This effect, a consequence of the nonadiabatic rotationless predissociation mechanism, is described using a coupled-channel treatment of the strongly Rydberg-valence-mixed (3)Pi(u) states. Significant J, e/f-parity, and sublevel dependencies observed in the isotopic F(v=1) rotational widths are found to derive from an indirect predissociation mechanism involving an accidental degeneracy with the E (3)Sigma(u)(-)(v=3) level, itself strongly predissociated by (3)Sigma(u)(-) Rydberg-valence interactions, together with L-uncoupling (rotational) interactions between the Rydberg components of the F and E states. Transitions into the E(v=3) level are observed directly for the first time, specifically in the O-18(2) spectrum.