Optical spectroscopy of V 635 Cassiopeiae / 4 U 0115 + 63

V635 Cas is the optical counterpart of the X-ray binary system 4U 0115+63. It was previously tentatively identified as a Be star based on its optical colours and the presence of Hα emission. Our observations indicate that it is an O9e star. This is the first direct determination of this star's optical spectral type. The presence of a hotter companion star may in part explain the large temporal variation observed in this system. Extreme variability was observed in 1992 February when both the Hα and a series of Paschen lines changed from emission to absorption. This was interpreted as a disk-loss event and it is the first time that it has been observed in this system. We use far red spectra of V635 Cas to probe the circumstellar disk, discussing the various line formation regions. The lines observed are consistent with a late type Oe star. The flux standard Hiltner 102 was also observed. Although it is classified as a B0 III star, we reclassify it as a O9.7 II star with a slight nitrogen enhancement.