The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (S-int,S-B) for the O-2 a (1)Delta(g) - X (3)Sigma(g)(-) (0,0) band. Einstein A-factors and radiative lifetimes for the O-2 a (1)Delta(g) v=0 state have been derived from the S-int,S-B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is S-int,S-B = 3.10 +/- 0.10 x 10(-24) cm molecule(-1), corresponding to an Einstein-A coefficient of A = 2.19 +/- 0.07 x 10(-4) s(-1) and a radiative lifetime of tau(rad) = 76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 226 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations.