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. Utilizes UDP-galactose solely as sugar donor. 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 stems, cauline leaves and siliques. Reduced levels of arabinogalactan proteins. Root hair defects. Reduced seed sets. Increased sensitivity to salt stress. Belongs to the glycosyltransferase 31 family.