A frequency-dependent electron spin resonance (ESR) study has been carried out of inherent Si dangling-bond-type point defects in thermal Si/SiO2 [P-b in (111)Si/SiO2, P-b0 and P-b1 in (100)Si/SiO2], admittedly generated as a result of mismatch induced interface strain. This has enabled the separation of the strain broadening component from other line broadening mechanisms, leading to a direct quantification of the associated interface stress. It is found that the technologically favored (100)Si/SiO2 interface exhibits generally more strain than typical for (111)Si/SiO2. However, the interface strain may be strongly reduced in both structures by appropriate postoxidation anneals. Additionally, information is gained on the spatial distribution of the defects: Strong evidence is provided that P-b0 and P-b1 in (100)Si/SiO2 are distributed in a different way than P(b)s at the (111)Si/SiO2 interface. Moreover, these distributions are found to be dependent on the thermal history of the sample, i.e., roughness of the interface layer.