The rotationally resolved absorption spectrum of the O-2 dimer involving the [O-2((1)Delta(g))(v=0)](2)<--[O-2((3)Sigma(g)(-))(v=0)](2) transition has been recorded near 632.6 nm by continuous wave Cavity Ring Down Spectroscopy in a supersonic slit jet expansion of pure O-2. A quadratic dependence of the absorption in the jet versus the stagnation pressure is observed. A rotational temperature of 12 K is derived from the (O-2)(2) rotational analysis. The high spectral resolution of the CW-CRDS measurements limited by the residual Doppler broadening in the jet and the low rotational temperature allow the first rotational analysis in this open-shell complex. The same spectrum was also recorded by Intracavity Laser Absorption Spectroscopy and the comparison of the performances of the two methods is discussed. Among more than 600 lines measured between 15 800 and 15 860 cm(-1) from the CW-CRDS spectrum, 40 were assigned to the P-R(0), (R)Q(0), and R-R(0) branches of two subbands associated with B-1(-)<-- A(1)(+) and A(1)(+)<--B-1(-) transitions between the ground and excited rovibrational levels, labeled following the G(16) permutation inversion representation. Forty five lines were assigned to P-P(2), (P)Q(2), and R-P(2) branches of two subbands associated with B-1(-)<-- A(1)(+) and A(1)(+)<-- B-1(-) transitions. The subbands centered at 15 808.401(49) [A(1)(+)<-- B-1(-)] and 15 813.134(37) cm(-1) [B-1(-)<-- A(1)(+)] for those arising from K=0, and at 15 812.656(20) [A(1)(+)<-- B-1(-)] and 15 818.277(35) [B-1(-)<-- A(1)(+)] when arising from K=2, are analyzed considering (O-2)(2) as a slightly asymmetric prolate top. The rotational analysis of the two K=0 subbands leads to very close values of the effective rotational constant, B-p=(B+C)/2, for both A(1)(+) and B-1(-) levels: 0.095 cm(-1) for the [O-2((3)Sigma(g)(-))(v=0)](2) lower states and 0.063 cm(-1) for the [O-2((1)Delta(g))(v=0)](2) excited states, in close agreement with theoretical values. The H geometry is confirmed as the most stable for the ground electronic singlet state. A distance between the two monomers of 6.1 a(0) and 7.5 a(0) is derived for the ground and excited singlet states. Similar results are obtained from the two K=2 subbands. A vibrational assignment is given for the two rotationally analyzed subbands (K=0) and proposed for the main features of the whole band. (C) 2000 American Institute of Physics. [S0021-9606(00)01614-7].