We present a first-principles study of ethylene oligomerization on single-site Ga 3+ catalysts supported on amorphous silica. Energetics of multiple oligomerization pathways are compared on two representative sites: 3CN and 4CN Ga. Although the 4CN sites may in principle promote the ethylene oligomerization cycle through a proton transfer cycle, they are not likely to be responsible for experimentally observed reactivity due to a high ethylene insertion barrier. The 3CN Ga site, however, yields a much more favorable energy landscape, which starts from a site activation process to form a Ga-alkyl species, followed by a Ga-alkyl-centered Cossee-Arlman mechanism.