An oil supply system may include a first hydraulic pump pumping oil to a low pressure portion through a first main line, a second hydraulic pump pumping oil to a high pressure portion through a second main line, first and second check valves that are respectively disposed at one side of the first and second main lines, a first convert valve supply the oil flowing the second main line to the low pressure portion depending on the pressure of oil that is transferred from a front side of the first check valve of the first main line, and a second convert valve supplying oil flowing the first main line to the high pressure portion depending on the pressure of oil that is transferred from a front side of the second check valve of the second main line.