Officials at the hurricane center said it was about 815 miles east-southeast of Bermuda and moving slowly west, but was expected to turn northwest soon.
The Tropical Storm Oscar has got upgraded into a Category 1 hurricane with maximum sustained winds of 120 kmph.
Forecasters say this hurricane season's 15th named storm is strengthening over the Atlantic Ocean, but it's no threat to land.
The Atlantic hurricane season runs from June 1 to November 30 each year, while the Eastern Pacific hurricane season runs from May 15 to November 30, the center said. A westward to west-southwestward motion with an increase in forward speed is expected today and tonight, followed by a reduction in speed on Sunday.
Sub-Tropical Storm Oscar has maximum sustained winds of 50 miles per hour (85 km/h), with higher gusts. The storm is continuing to transition from a hybrid storm to a full-fledged tropical system on Saturday night. It will weaken and become a powerful extratropical low over the north-central Atlantic by late Wednesday, forecasters said.