Nonlinear frequency conversion enables coherent light generation in spectral regions that conventional lasers cannot reach directly and is foundational to the generation of squeezed states and correlated photon pairs. Implementing these nonlinear processes in microresonators lowers the required optical power by orders of magnitude but requires two conditions to be met simultaneously across wide optical bandwidths: phase matching between the interacting fields, and an appropriate coupling condition to the resonator both in the fundamental and second harmonic band. While the former can be addressed by quasi-phase matching through periodic poling, coupling two widely separated spectral bands to the resonator is nontrivial because of the wavelength dependence of mode overlap. Here, we demonstrate parametric oscillation and efficient second-harmonic generation in periodically poled racetracks on thin-film lithium tantalate on insulator. The proposed resonators feature bus-ring coupling engineered independently in the two spectral bands via a long synchronous directional coupler. Quasi-phase matching is achieved using sidewall poling that is also wavelength-calibrated and locally adapted to compensate for thickness nonuniformities. Together, these two design elements enable second-order parametric oscillation with an on-chip threshold of 181 uW and 9.8% conversion efficiency, with signal and idler tunable over tens of nanometers by a few hundred megahertz of pump detuning. Our design strategy can be flexibly applied to efficient second-harmonic generation, reaching 25% absolute conversion efficiency at milliwatt pump powers. These results show how the coupling condition is a design variable in second-order nonlinear resonators and establish thin-film lithium tantalate as a competitive platform for efficient and low-power frequency conversion.