An oil boom is disclosed which comprises two rows of upper and lower stream side; a flexible base sheet connected across base portions of the two rows of floats and hanging down therefrom into the water in a gently curved shape. A plurality of inlet openings are arranged near the upper stream side float and extending in its lengthwise direction; an inflow guide plate made integral with the flexible base sheet and arranged below the inlet openings. A plurality of outlet openings are arranged in a rear portion of the flexible base sheet and extending in its lengthwise direction, the total opening area of the outlet openings per unit length of the flexible base sheet being larger than that of the inlet openings. Thus the oil boom may be utilized not only to dam and collect and oil slick floating on water, but also as an oil collecting region to separate the oil slick from the water.