<p>An oil pump oil supply system comprises an oil pump for providing a pressure source and a first oil supply passage communicated with an oil pan and a crankcase. The oil pump is arranged inside a pump chamber, and lubricating oil enters the crankcase through the first oil supply passage. The oil pump oil supply system further comprises a second oil supply passage communicated with a rocker arm chamber and the pump chamber. The oil pump is connected in series in the first oil supply passage and the second oil supply passage. With the oil pump oil supply system, a four-stroke engine is capable of discharging the lubricating oil inside an oil-gas separation chamber rapidly when the four-stroke engine is slant or upside down during use.</p>
