OASIS (Orbiting Astronomical Satellite for Investigating Stellar Systems) is a space-based observatory with a large inflatable primary reflector that will perform high spectral resolution observations at terahertz frequencies. An inflatable metallized polymer membrane serves as the primary antenna with large photon collecting area, followed by aberration correction mirror pair that enables a large field of regards of 0.1 degrees while achieving diffraction limited performance over a wide terahertz wavelength ranging from 80 μm to 660 μm. An analytical model is developed to define a solution space based on the profile of primary reflector which is a function of pressure. The photon collecting area, size and weight of the correction mirror pair, and optical aberrations are governed by a 1st order power arrangement of the telescope and is a function of base radius and clear aperture of the primary reflector. Based on the parametric design study, the figure of merit for the profile of the primary reflector is discussed and a baseline design satisfying the scientific and system requirements is proposed.