A high-resolution threshold photoelectron spectrum of molecular oxygen has been studied between 18 and 24 eV using the penetrating field technique and a synchrotron radiation source. Nine ionic states are observed, and three of them for the first time. Higher vibrational levels of the b (4) Sigma(g)(-) state and the (2) Phi(u) state are also studied. Furthermore, the doublet structure of the (2) Phi(u) state is resolved for the first time. Two new states with quartet multiplicity are assigned to the (4) Pi(g) and the (4) Pi(u) states which dissociate to the limit, [O(P-3)+O+(D-2)], at 22.059 eV. Also we tentatively assign another newly observed band to the (2) Pi(u) state. The long vibrational progression of the 3 (2) Pi(u) state is clearly observed in the region 23.3-23.8 eV with doubler splitting which increases with increasing of the vibrational quantum number.