The orbit-response matrix (ORM) of an accelerator is obtained by measuring the closed orbit deviation produced by an individual excitation of each correction coil. In theory, this matrix only depends on the beta functions and betatron phases at the beam position monitors (BPMs) andat the correction coils. By solving this nonlinear dependence by an iterative method, we have measured the Twiss functions of the HERA rings. Furthermore one can fit optical errors like quadrupole strength or roll angles to obtain the field deviations that produce a discrepancy between themeasured and the theoretical ORM. Both methods are useful tools for finding errors of BPMs and of correction coils. While both methods, especially the second one, have been used at several circular accelerators, they have been found to become less useful and less accurate with an increasing size of the accelerator. Therefore they have never been intensively applied to an accelerator as large as HERA. Here we will report that the ORM analysis has nevertheless been found to be very useful for checking HERA’s BPM system, for finding erroneous correction coils and for finding erroneous quadrupole settings.