The WO stars form a subgroup of the WR class characterized by a strong O VI emission at 382 nm (Sanduleak 1971; Barlow & Hummer 1982). In 1989–90 we observed intermediate resolution spectra of the four Sanduleak WO stars and of their surrounding nebulosities using the Bologna-Loiano 1.52m and ESO-La Silla 1.52m and 2.2m telescopes (e.g., Polcaro et al. 1991, 1992, 1994 in preparation). Fig. 1 shows that the spectra are similar but with some striking differences among the four stars. They can be arranged in a sequence in order of increasing ionization: Sand 2, Sand 1, Sand 5, Sand 4 (see Table 1). Sand 4 is a binary system, and the continuum of the O4 companion dominates over that of the WO star. In the other objects, the reddening-corrected continuum can be fitted by a 80 000 K BB, in agreement with the high ionization level of their emission spectra.