A method for separating water from oil, the method comprising: - introducing oil that contains water into a degassing device (31, 33), wherein the degassing device (31, 33) contains an upper volume (31A, 33A) filled with gas and a lower volume filled with oil up to an oil level, - introducing gas into the oil to form gas bubbles rising through the oil in the degassing device (31, 33), while the gas bubbles are absorbing water vapor which, together with the gas in the bubbles, is released to the upper volume (31A, 33A) for discharge from the degassing device (31, 33) through an exhaust system connected to the upper volume (31A, 33A), characterized in that the degassing device (31,33) comprises a first and a second degassing device, each having a lower volume (31B, 33B) containing oil and being mutually fluid-flow connected, and each having an upper volume (31A, 33A) containing gas and being mutually fluid-flow connected and connected to the exhaust system; wherein the method comprises - raising the oil level in the second degassing device from a predetermined lower level to a predetermined upper level by adding oil, which decreases the upper volume (31A, 33A) for the gas and presses gas from the upper volume (31A, 33A) out of the upper volume (31A, 33A) through the exhaust system; - preventing backflow of gas from the exhaust system into the upper volume (31A, 33A); - when the oil level reaches the predetermined upper level, increasing the upper volume (31A, 33A) by lowering the oil level in the second degassing device to the lower predetermined level and thereby pulling gas from the upper volume of the first degassing device into the upper volume of the second degassing device and forcing the gas in the upper volumes (31A, 33A) of the first and the second degassing devices (31, 33) to expand, and thereby decrease the gas pressure; and - using the decreased gas pressure for additional extraction of the gas bubbles and water from the oil in the first and second degassing devices (31, 33). - introducing oil that contains water into a degassing device (31, 33), wherein the degassing device (31, 33) contains an upper volume (31A, 33A) filled with gas and a lower volume filled with oil up to an oil level, - introducing gas into the oil to form gas bubbles rising through the oil in the degassing device (31, 33), while the gas bubbles are absorbing water vapor which, together with the gas in the bubbles, is released to the upper volume (31A, 33A) for discharge from the degassing device (31, 33) through an exhaust system connected to the upper volume (31A, 33A), characterized in that the degassing device (31,33) comprises a first and a second degassing device, each having a lower volume (31B, 33B) containing oil and being mutually fluid-flow connected, and each having an upper volume (31A, 33A) containing gas and being mutually fluid-flow connected and connected to the exhaust system; wherein the method comprises - raising the oil level in the second degassing device from a predetermined lower level to a predetermined upper level by adding oil, which decreases the upper volume (31A, 33A) for the gas and presses gas from the upper volume (31A, 33A) out of the upper volume (31A, 33A) through the exhaust system; - preventing backflow of gas from the exhaust system into the upper volume (31A, 33A); - when the oil level reaches the predetermined upper level, increasing the upper volume (31A, 33A) by lowering the oil level in the second degassing device to the lower predetermined level and thereby pulling gas from the upper volume of the first degassing device into the upper volume of the second degassing device and forcing the gas in the upper volumes (31A, 33A) of the first and the second degassing devices (31, 33) to expand, and thereby decrease the gas pressure; and - using the decreased gas pressure for additional extraction of the gas bubbles and water from the oil in the first and second degassing devices (31, 33).