The 4s-3d and 5s-4d Rydberg complexes of diatomic oxygen have been studied by (2 + 1) resonance-enhanced multiphoton ionization of the X 3Σg- ground state of O2. We have located and identified at least two vibrational levels of each of the following states: Three of four expected 4sσ Π states; all four expected 5sσ Π states; 18 of 22 expected 3d states (with only the states of the 3dσ orbital remaining unobserved); and 5 of the 10 predicted 4dπ states. State assignments were assisted by the following: the results of rotational cooling and laser polarization experiments which facilitated the rotational analysis, band positions, band intensities, and parameterized calculations. The experimentally determined state locations are compared with the state locations obtained from ab initio calculations. We have carried out isotope experiments and rotational linewidth analysis to study in some detail the mixing between the Rydberg states and the repulsive valence states as well as the mixing between the Rydberg states themselves. We conclude that direct predissociation dominates indirect predissociation as a dissociative mechanism, but there is evidence of Δv;≠0 interactions which perturb the rotational structure of the 3dπ Σ and Δ states. The relative intensities of the states detected are found to span a range in excess of 104 with the nsσ Π states being the weakest and the ndπ Σ states being the strongest. Photoionization of the ndπ Σ states appears to be most affected by the shape resonance in the continuum. Our measurements confirm the expectation that many of the properties of the Rydberg states in the same series scale as (n*)-3.