Reflectance anisotropy spectroscopy (RAS) was used to study metalorganic vapor phase epitaxial growth of GaAs(001). In previous studies, GaAs(001) surfaces have been identified as c(4x4)-like under pregrowth arsenic stabilized conditions. During growth the RAS spectra showed significant differences with respect to the pregrowth spectra. Moreover, the RAS signal at a fixed photon energy has been shown to exhibit an oscillatory behavior when growth is initiated. The period of these oscillations has been identified as the time needed for the growth of exactly one monolayer. In this study, the dependence of the oscillations (amplitude and period) on temperature and trimethylgallium (TMGa) partial pressure is investigated and discussed together with the corresponding RAS spectra taken at V/III ratios between 5 and 300. Three different types of RAS spectra are identified during growth. Firstly, at high temperatures and low TMGa partial pressures, the surface anisotropy is nearly the same as in the c(4X4)-like pregrowth state, with a minimum at 2.5 eV (type I spectra). This indicates that the surface during growth is essentially covered with a double layer of As. Secondly, at low temperatures or high TMGa partial pressures a different structure with a sharp minimum at 2.65 eV is observed (type III spectra). This might be related to a surface largely covered with TMGa fragments. Finally, intermediate between these extreme epitaxial conditions, RAS spectra with a minimum at 2.0 eV and nearly vanishing anisotropy around 2.5 eV are found. This indicates that the As dimers of the pregrowth surface have largely disappeared and a new surface structure, possibly consisting of Ga dimers, is now present. Oscillations have their maximum amplitude at the boundary between type II and type III spectra in a p(TMGa)-T diagram. At this boundary the growth rate becomes nonlinear and starts to saturate with increasing pressure of TMGa.