The Simons Observatory (SO) is a new cosmic microwave background (CMB) experiment being built on Cerro Toco in Chile, with observations expected to begin in the early 2020s. The SO collaboration, comprising over 200 scientists from 40 institutions, aims to measure the temperature and polarization anisotropy of the CMB in six frequency bands centered at 27, 39, 93, 145, 225, and 280 GHz. The initial configuration will include three small-aperture 0.5-meter telescopes and one large-aperture 6-meter telescope, with a total of 60,000 cryogenic bolometers. Key scientific goals of the SO include characterizing primordial perturbations, measuring the number of relativistic species and neutrino mass, testing deviations from a cosmological constant, improving understanding of galaxy evolution, and constraining the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping approximately 10% of the sky to a white noise level of 2 μK-arcmin in the combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, \( r \), at a target level of \(\sigma(r) = 0.003\). The large aperture telescope will map approximately 40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in the combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope (LSST) sky region and partially with the Dark Energy Spectroscopic Instrument (DESI). The paper presents a baseline model for the SO instrument performance, including detector noise, frequency bands, and angular resolution, and discusses the anticipated properties of temperature and polarization maps. It also provides forecasts for various cosmological signals, such as large-scale \(B\)-modes, small-scale damping tails, gravitational lensing, primordial bispectrum, thermal and kinematic Sunyaev-Zel'dovich effects, and extragalactic sources. The forecasts are based on assumptions about atmospheric and galactic foreground emission, sky coverage, and foreground cleaning methods. The results have been used to optimize experimental design choices, particularly aperture sizes, angular resolutions, detector division between large and small aperture telescopes, frequency band selection, and detector division within frequency bands.The Simons Observatory (SO) is a new cosmic microwave background (CMB) experiment being built on Cerro Toco in Chile, with observations expected to begin in the early 2020s. The SO collaboration, comprising over 200 scientists from 40 institutions, aims to measure the temperature and polarization anisotropy of the CMB in six frequency bands centered at 27, 39, 93, 145, 225, and 280 GHz. The initial configuration will include three small-aperture 0.5-meter telescopes and one large-aperture 6-meter telescope, with a total of 60,000 cryogenic bolometers. Key scientific goals of the SO include characterizing primordial perturbations, measuring the number of relativistic species and neutrino mass, testing deviations from a cosmological constant, improving understanding of galaxy evolution, and constraining the duration of reionization. The small aperture telescopes will target the largest angular scales observable from Chile, mapping approximately 10% of the sky to a white noise level of 2 μK-arcmin in the combined 93 and 145 GHz bands, to measure the primordial tensor-to-scalar ratio, \( r \), at a target level of \(\sigma(r) = 0.003\). The large aperture telescope will map approximately 40% of the sky at arcminute angular resolution to an expected white noise level of 6 μK-arcmin in the combined 93 and 145 GHz bands, overlapping with the majority of the Large Synoptic Survey Telescope (LSST) sky region and partially with the Dark Energy Spectroscopic Instrument (DESI). The paper presents a baseline model for the SO instrument performance, including detector noise, frequency bands, and angular resolution, and discusses the anticipated properties of temperature and polarization maps. It also provides forecasts for various cosmological signals, such as large-scale \(B\)-modes, small-scale damping tails, gravitational lensing, primordial bispectrum, thermal and kinematic Sunyaev-Zel'dovich effects, and extragalactic sources. The forecasts are based on assumptions about atmospheric and galactic foreground emission, sky coverage, and foreground cleaning methods. The results have been used to optimize experimental design choices, particularly aperture sizes, angular resolutions, detector division between large and small aperture telescopes, frequency band selection, and detector division within frequency bands.