The storm had maximum sustained winds of 95 kph (60 mph) Tuesday evening, and forecasters warned it could trigger dangerous floods by dumping as much as 25 centimeters (10 inches) of rain in some parts of the Dominican Republic and neighboring Haiti.
Olga struck nearly two weeks after the official end of the Atlantic hurricane season. It is only the 10th named storm to develop in the month of December since record keeping began in 1851, according to the U.S. National Hurricane Center in Miami.
"It's not completely unusual to have a storm form in December," said Daniel Brown, a hurricane specialist at the center, who noted that three named storms have formed after Nov. 30 since 2003.
At 10 p.m. EST (0300 GMT Wednesday), Olga was centered about 90 kilometers (55 miles) northwest of Santo Domingo, the Dominican capital, and was continuing west toward Haiti, the hurricane center said.
The storm was passing along the southern Dominican coast, threatening southwestern areas that were hardest hit by Tropical Storm Noel six weeks ago. At least 87 fatalities in the country were blamed on Noel, the deadliest storm of the 2007 Atlantic hurricane season.
Authorities evacuated 22 communities in eastern provinces to prepare for the storm.
"Every civil defense agency has been activated," emergency services spokesman Luis Luna Paulino said.
In neighboring Haiti, the government warned people in coastal areas to prepare for evacuations and urged fishermen to remain in port, said Marie Alta Jean-Baptiste, head of the country's civil protection department.
Olga was tracking westward at about 24 kph (15 mph), and the center of the storm was expected to remain over land for much of the next 24 hours. Forecasters predicted it would gradually weaken into a tropical depression as its center crosses Haiti and continues across the Caribbean between Cuba and Jamaica .
The storm passed Puerto Rico on Tuesday night, knocking out electrical service to 79,000 people and water to 144,000.
A tropical storm warning was in effect for the southeastern Bahamas and the Turks and Caicos Islands, meaning tropical storm conditions are expected there within 24 hours.
Forecasters said Olga's stronger winds on Tuesday evening indicated that it had transitioned from a subtropical storm to a tropical storm over the course of the day.
Subtropical storms often form over cooler waters than those needed by tropical storms, which usually have their strongest winds located close to a well-defined, spinning center.
Olga will be included in the tally for the 2007 hurricane season, bringing the number of named storms to 15, including six hurricanes. The next season begins June 1.