The effect of oxygen segregation at grain boundaries on the deformation of 1 wt% boron (B)- and carbon (C)-doped beta-silicon carbide (B, C-doped SiC) was investigated by compression testing at 2073 K. We studied the deformation of sinter-forged B, G doped SiC (sinter-forged SiC), which contained the minimum amount (0.07 wt%) of oxygen as an impurity, and that of hot isostatically pressed B, C-doped SiC (HIPed SiC), which contained 1 wt% oxygen. Oxygen was detected at grain boundary in HIPed SiC by energy-dispersive X-ray spectroscopy, but it was not detected in sinter-forged SiC. The strain rate of sinter-forged SiC was one order of magnitude lower than that of HIPed SiC at the same grain size. The grain growth rate of sinter-forged SiC was lower than that of HIPed SiC also. These results suggest that the oxygen segregation at grain boundaries, together with boron segregation, promoted the grain-boundary diffusion in B, C-doped SiC. But, the oxygen segregation without boron was less effective in promoting deformation than the boron segregation without oxygen.