Oligomerization processes include the steps of introducing a monomer containing a C3 to C30 olefin and a chemically-treated solid oxide into a reaction zone, and oligomerizing the monomer to form an oligomer product in the reaction zone. Fluorided silica-coated alumina and fluorided-chlorided silica-coated alumina are illustrative chemically-treated solid oxides that can be used in the oligomerization processes.