The characteristics of noncollinear phase matching and quasi-phase-matching in the THz-wave parametric oscillator (TPO) are investigated. The expression of the effective parametric gain length under the condition of noncollinear phase matching configuration is deduced. The relationship between the poling period of periodically poled LiNbO₃ crystal and the generated THz frequency under the condition of quasi-phase-matching configuration is analyzed. Based on the analyses above we propose a new TPO configuration which ensures the three mixing waves interact collinearly. The effects of operation temperature on phase matching are analyzed.