High-power, high-directivity terahertz radiation is generated by resonant tunneling diode (RTD) oscillator arrays formed by vertically integrated twisted slot antennas. Enhanced combined radiation is achieved through the antiphase coupling between adjacent RTDs. Tailored electrode shapes mitigate electric field cancellation to further increase output power. Experimental results confirm coherent oscillation in a 2x2 array at frequencies up to 513 GHz and output power up to 0.4 mW. Increased array scaling is predicted to improve directivity with a low-diffusion quasi-plane wavefront.