We present first results of a long-term study: Searching for OB-type runaway stars inside supernova remnants (SNRs). We identified spectral types and measured radial velocities by optical spectroscopic observations and we found an early type runaway star inside SNR S147. HD 37424 is a B0.5V-type star with a peculiar velocity of 74 +/- 8 kms(-1). Tracing back the past trajectories via Monte Carlo simulations, we found that HD 37424 was located at the same position as the central compact object, PSR J0538+2817, 30 +/- 4 kyr ago. This position is only similar to 4 arcmin away from the geometrical centre of the SNR. So, we suggest that HD 37424 was the pre-supernova binary companion to the progenitor of the pulsar and the SNR. We found a distance of 1333(-112)(+103) pc to the SNR. The zero-age main sequence progenitor mass should be greater than 13 M-circle dot. The age is 30 +/- 4 kyr and the total visual absorption towards the centre is 1.28 +/- 0.06 mag. For different progenitor masses, we calculated the pre-supernova binary parameters. The Roche lobe radii suggest that it was an interacting binary in the late stages of the progenitor.