The electric-dipole forbidden a(1)Delta (g) - X(3)Sigma (-)(g) (0,0) band of gas-phase O-2 has been studied in absorption at wavelengths around 1.27 mum using Fourier transform spectroscopy and a long-path absorption cell. Experiments were conducted at temperatures of 294, 243, and 200 K and at pressures in the range 140-750 Torr. Both pure O-2 and a mixture of 21% O-2/79% N-2 were studied, and line widths, integrated line intensities, and integrated absorption intensities (AIs) for the (0,0) vibrational band were measured. Integrated AIs were found to be independent of temperature, pressure, and gas composition, and the recommended value for the vibrational band from the current study is S-int = 3.210(15) x 10(-24) cm molecule(-1) (1 sigma error) for pure O-16(2), corresponding to an Einstein A-coefficient of A = 2.256(10) x 10(-4) s(-1). The effect of including other oxygen isotopomers is to increase the integrated AI value for this origin band to S-int = 3.226(15) x 10(-24) cm molecule(-1) Widths of individual spectroscopic lines decrease with increasing molecular rotational quantum number. The temperature dependence of Lorentzian line width components, gamma (T) (fwhm), is well represented by the expression gamma (T) = gamma (294K)(T/294 K)(-0.85). The effects of pressure broadening of lines by N-2 are indistinguishable from those for pure O-2 at the same temperature and pressure.