Is there an O1 or O0 star? Okay, we've seen the super hot Wolf-Rayet stars, especially WR 102 and 142, and the &quot;slash stars,&quot; many of which are early O (O2-4.5/WN). We know the temperatures of these Wolf Rayet stars. So if we know that, what would their spectral types be in the standard Morgan-Keenan spectral classification (i.e. O2V or B0Ia+), and are there known stars with spectral class earlier than O2?
This is what Wikipedia says about it:

When the MKK classification scheme was first described in 1943, the only subtypes of class O used were O5 to O9.5. The MKK scheme was extended to O9.7 in 1971 and O4 in 1978, and new classification schemes that add types O2, O3, and O3.5 have subsequently been introduced.

It references the paper A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Thus, a new earliest spectral type has been defined.

So unless that information is outdated, there's no such thing as O1 or O0.