All work must be authorized, so work for a side company must be work authorized, although it's possible to hold both full-time and part-time H-1Bs or O-1 or TNs

All work must be authorized, so work for a side company must be work authorized, although it's possible to hold both full-time and part-time H-1Bs or O-1 or TNs with multiple companies. The criteria for O-1 classification are listed on USCIS's web site - too much to note here - but as a general rule, if a company is doing well, then it's relatively easy to get an O-1.