A 26-year-old DRC ranger died "following an attack carried out by armed men against a team of eco-rangers who were on a surveillance missions near the 'Mondial,' an illegal mining area" in the center of the park, the RFO said in a statement.
Another ranger was wounded and an assailant was captured, it said.
The RFO, located in Ituri province, is a World Heritage site and haven for the okapi, an endangered deer-like animal that is related to the giraffe.
Only 30,000 okapi are left in the wild, of which around 5,000 live in the RFO.
The biggest country in sub-Saharan Africa, the Democratic Republic of Congo has world famous wildlife reserves.
But those located in its troubled east struggle with illicit mining, deforestation and poaching, abetted by armed groups which plague the region.
In May, five guards and a civilian employee of the Virunga National Park, a biodiversity hotspot and sanctuary for the mountain gorilla, were killed by armed groups.
DRC Wildlife Ranger Killed
FILE: In this March 18, 2010 photo, Mbuti pygmy men await the start of the day's hunt, in the Okapi Wildlife Reserve outside the town of Epulu, DRC. The pygmies are not being blamed for the recent fatal attack on the wildlife ranger.
BENI, DEMOCRATIC REPUBLIC OF CONGO — A ranger was killed and a second wounded when their team came under attack near an illegal mine on Tuesday at the Okapi Wildlife Reserve (RFO) in the DRC's northeast. The incident was reported Thursday by officials.