The absolute rate constant for the reaction HCO + O-2 has been measured dyer the temperature range T = 200-398 K in a discharge flow system at a pressure of 1 Torr (He); This study represents the first measurement of:this rate constant at temperatures below T = 298 K. The decay of the HCO radical in the presence of excess Oz was followed by collision-free sampling photoionization mass spectrometry. The results were as follows: k(1)(200 K) = 5.3 +/- 0.8, k(1)(222 K) = 4.8 +/- 0.7, k(1)(250 K) = 4.4 +/- 0.6, k(1)(298 K) = 4.0 +/- 0.6, k(1)(398 K) = 4.5 +/- 0.7 in units 10(-12) cm(3) molecule(-1) s(-1). Over the range T = 200-298 K, the data suggest a slight negative temperature dependence and may be represented by the Arrhenius expression k(1) = (2.2 +/- 0.2) x 10(-12) exp[(170 +/- 22)/T] cm(3) molecule(-1) s(-1). For the related reaction HCO + NO at T = 298 K we measure k(2) = (1.3 +/- 0.2) x 10(-11) cm(3) molecule(-1) s(-1). The results are compared with previous direct measurements of k(1) and k(2) and with recent theoretical calculations. For the HCO + O-2 reaction there are larger than expected differences between the measured values for what appears to be a relatively simple and well studied reaction. A recent theoretical expression for k(1), recommended for the region T = 300-3000 K, can be made to be in reasonable agreement with most of the results from direct, temperature-dependent studies between T = 200-713 K by an extrapolation to T = 200 K and by multiplying the theoretical expression by a minor uniform scaling factor of 0.80. Our result for HCO + NO at T = 298 K is in excellent agreement with all except one of the previous measurements of this rate constant.