Monoterpene synthase involved in the biosynthesis of (E)-beta-ocimene as the major product and trace amounts of (Z)-beta-ocimene. Can only accept geranyl diphosphate as substrate. (2E)-geranyl diphosphate = (E)-beta-ocimene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Secondary metabolite biosynthesis; terpenoid biosynthesis. Highly expressed in leaves, stems and disk florets. Detected in roots. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsb subfamily.