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We theoretically analyze and experimentally optimize an O-band mixed-cascaded multiwavelength phosphosilicate Raman fiber laser (MRFL). The theoretical analyses show that the output power is insensitive to the variation of Raman fiber length when it exceeds the optimal length. However, the pump threshold, slope conversion efficiency, and output power of the mixed-cascaded MRFL strongly depend on the output coupling ratio of the output optical coupler. When a 1064-nm Yb³⁺-doped double-clad fiber laser is used to pump a section of phosphosilicate fiber (PSF) in a mixed-cascaded Raman cavity, both the Raman gains of P₂O₅ and SiO₂ in the fiber are utilized, and multiwavelength lasing around 1320 nm with a wavelength spacing of 0.8 nm has been obtained. The different coupling output ratios and Raman fiber lengths are used to optimize the output performances of the O-band mixed-cascaded MRFL. The maximum output power of the O-band MRFL is 1.3 W with the coupling output ratio of 80% and the PSF length of 1 km. The experimental results are in good agreement with the theoretical ones.