Porous is comprised of nanometer-scale crystallites of Si which, on exposure to the atmosphere, become surrounded by thin oxide layers. It has been proposed that the red room temperature luminescence in this material is primarily due to defects at or near the interface between the crystallites and the thin oxide. We report here the observation of two electron spin resonance (ESR) active oxygen-centric defects; one in nominally unoxidized porous Si and the second in material which has been oxidized for a short period of time. The first is a variant of the thermal donors observed in crystalline Si and the second is the EX center observed in thin layers of SiO2 on crystalline Si. These two centers are thought to have similar cores, a Si vacancy surrounded by four Si-O linkages, with the primary difference in the surrounding lattice (Si vs SiO2). The ESR intensity of the EX center is related to the photoluminescence intensity and may be directly involved in the luminescence process.