Arcus is a mission concept for a next generation X-ray grating spectrometer. It will offer spectral resolution (λ/Δλ) greater than 2000 combined with over 500 cm2 of effective area in the 2.1-2.4 nm bandpass. These capabilities will elucidate the cycle of baryonic matter in and out of galaxies, the means by which supermassive black holes influence their surroundings, and the early formation and evolution of solar systems. We present the overall optical design of the mission, which features four arrays of silicon pore optics modules with four matching arrays of off-plane reflection grating modules. These optics disperse the incident X-rays over the 12 m focal length in four separate conical diffraction patterns onto CCD arrays at the focal plane. Each array of optics is an azimuthal sub-aperture of the typical Wolter telescope design, enabling enhanced spectral resolution due to an asymmetric point spread function. The theoretical spectral resolution, effective area, and alignment tolerances have been determined via raytrace modeling.