Catalyzes the reaction that attaches fucose through an O-glycosidic linkage to a conserved serine or threonine residue found in the consensus sequence C2-X(4,5)-[S/T]-C3 of EGF domains, where C2 and C3 are the second and third conserved cysteines (PubMed:12909620). Specifically uses GDP-fucose as donor substrate and proper disulfide pairing of the substrate EGF domains is required for fucose transfer (By similarity). Plays a crucial role in Notch signaling (PubMed:12917292, PubMed:12909620, PubMed:17329366). Initial fucosylation of Notch/N by O-fut1 generates a substrate for Fringe/Fng, an acetylglucosaminyltransferase that can then extend the fucosylation on the Notch EGF repeats (PubMed:12909620). This extended fucosylation is required for optimal ligand binding and canonical NOTCH signaling induced by Delta/Dl or Serrate/Ser (PubMed:12909620, PubMed:12917292). Also required for the transport of Notch/N to the early endosome (PubMed:17329366). 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. Interacts with N (via its EGF domains); the interaction is required for the endocyctic transport of N. Expressed both maternally and zygotically. Belongs to the glycosyltransferase 65 family. Truncated N-terminus.