Possesses hydroxyproline O-galactosyltransferase activity. Transfers galactose from UDP-galactose to hydroxyproline residues in the arabinogalactan proteins (AGPs). Is specific for AGPs containing non-contiguous peptidyl hydroxyproline residues. The addition of galactose onto the peptidyl hydroxyproline residues in AGP core proteins represents the first committed step in arabinogalactan polysaccharide addition. AGP glycans play essential roles in both vegetative and reproductive plant growth. Protein modification; protein glycosylation. Expressed in roots, rosette leaves, cauline leaves, stems, flowers and siliques. Reduced levels of arabinogalactan proteins. Belongs to the glycosyltransferase 31 family.