Transfers mannosyl residues to the hydroxyl group of serine or threonine residues. The 4 members of the TMTC family are O-mannosyl-transferases dedicated primarily to the cadherin superfamily, each member seems to have a distinct role in decorating the cadherin domains with O-linked mannose glycans at specific regions. Also acts as O-mannosyl-transferase on other proteins such as PDIA3. a dolichyl beta-D-mannosyl phosphate + L-seryl-[protein] = 3-O-(alpha-D-mannosyl)-L-seryl-[protein] + a dolichyl phosphate + H(+) a dolichyl beta-D-mannosyl phosphate + L-threonyl-[protein] = 3-O-(alpha-D-mannosyl)-L-threonyl-[protein] + a dolichyl phosphate + H(+) Protein modification; protein glycosylation. Belongs to the TMTC family.