We present the results of selective-area growth of GaN nanowires by molecular beam epitaxy on patterned SiOx/Si substrates without using seed layers. The morphological and optical properties of selectively grown GaN nanowires are compared to the properties of GaN nanowires grown on the amorphous SiOx layer. The experimental results show that the selectivity of GaN nanowires is achieved at a substrate temperature of 825 C which is the lower limit for the selective-area growth of GaN nanowires on SiOx/Si substrates. The study of the photoluminescence spectra of the grown nanowires, measured at 77 K show an emission line at 3.47 eV, which corresponds to strain-free GaN.