Catalyzes the interconversion of ornithine to glutamate semialdehyde. a 2-oxocarboxylate + L-ornithine = an L-alpha-amino acid + L-glutamate 5-semialdehyde Amino-acid biosynthesis; L-proline biosynthesis; L-glutamate 5-semialdehyde from L-ornithine: step 1/1. Belongs to the class-III pyridoxal-phosphate-dependent aminotransferase family. OAT subfamily.