A protein glycosylation process in which a carbohydrate or carbohydrate derivative unit is added to a protein via the hydroxyl group of peptidyl-serine, peptidyl-threonine, peptidyl-hydroxylysine, or peptidyl-hydroxyproline, or via the phenol group of peptidyl-tyrosine, forming an O-glycan. [ https://www.worldcat.org/search?q=bn%3A0879695595 https://proteininformationresource.org/cgi-bin/resid?id=AA0154 https://proteininformationresource.org/cgi-bin/resid?id=AA0153 https://proteininformationresource.org/cgi-bin/resid?id=AA0155 https://proteininformationresource.org/cgi-bin/resid?id=AA0157 https://proteininformationresource.org/cgi-bin/resid?id=AA0212 ]