An oil supply system may include a first hydraulic pump, a second hydraulic pump, first and second check valves, a first convert valve, and a second convert valve. The first hydraulic pump pumps oil to a low pressure portion through a first main line. The second hydraulic pump pumps oil to a high pressure portion through a second main line. The first and second check valves are respectively disposed at one side of the first and second main lines. The first convert valve supplies 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. The second convert valve supplies 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.