SAN JUAN (Reuters) - Hurricane Omar strengthened on Wednesday as it bore down on Puerto Rico and the small islands of the northeastern Caribbean, and could be a "major" storm by the time it reaches them, U.S. forecasters said. The 15th tropical cyclone of a busy Atlantic hurricane season formed north of the Dutch island of Curacao on Tuesday, briefly preventing Venezuela from loading tankers with crude oil and knocking out power at the OPEC nation's 200,000 barrel-per-day Puerto La Cruz refinery. The U.S. National Hurricane Center predicted Omar would grow into a Category 2 storm on the five-step Saffir-Simpson...