Catalyzes the reaction that attaches fucose through an O-glycosidic linkage to a conserved serine or threonine residue in the consensus sequence C1-X-X-S/T-C2 of thrombospondin type I repeats (TSRs) where C1 and C2 are the first and second cysteines of the repeat, respectively (PubMed:15233996, PubMed:26854667). O-fucosylates members of several protein families including the ADAMTS superfamily and the thrombospondin (TSP) and spondin families (By similarity). GDP-beta-L-fucose + L-seryl-[protein] = 3-O-(alpha-L-fucosyl)-L-seryl-[protein] + GDP + H(+) GDP-beta-L-fucose + L-threonyl-[protein] = 3-O-(alpha-L-fucosyl)-L-threonyl-[protein] + GDP + H(+) Protein modification; protein glycosylation. Mainly located in the endoplasmic reticulum. Expressed in the anterior part of embryos, in the hypodermal and neuronal cells of the head. Expressed at different levels in a variety of cell types after hatching, including neuronal, hypodermal, muscle, intestinal, and somatic gonadal cells. Expressed in the nerve ring around the pharynx, in dorsal and ventral nerve cords, intestine, and a variety of hypodermal cells of L1-L3 larvae. Expressed in gonadal sheath cells, spermatheca, and tissues surrounding the vulva of adult hermaphrodites, and in the body wall muscle and hypodermal cells of adults of both sexes. Expressed starts at morphogenetic embryonic stages and continues throughout development. Belongs to the glycosyltransferase 68 family.