In this paper, we report experimental evidence of forward optical parametric oscillations (OPOs) with multi frequencies conversion processes in a spectral range of 400 nm–1700 nm using nonlinear photonic crystals of periodically poled LiTaO3 without external mirrors. The highly polished end-facets of the samples act as a Fabry–Perot cavity mirrors making the device simple and compact. Oscillation thresholds were found of 97.6 MW cm−2 and 160 MW cm−2 for the 1D and 2D samples, respectively. Moreover, at pump energies above the threshold, optical oscillations of multiple frequencies due to cascaded interactions established within the samples are obtained. In particular, we point-out the generation of two pairs of signal and idler (OPO1 and OPO2) in the same nonlinear photonic crystal. It is found that the second pair OPO2 appears with different quasi-phase matching conditions induced by a local refractive index variation. Our analysis indicates that this index variation results from a competing effect between quadratic cascading and electro-optic induced refractive index changes.