What structure supplies oxygenated blood to the heart muscle?

Coronary arteries. The orifice of the coronary arteries are located on proximal part of the ascending aorta, and there are two orifices branching into the left and right coronary arteries.

The marginal, anterior and ventricular arteries provide the cardiac muscle with oxygenated blood.

The coronary arteries are the major vessels bringing blood to the heart muscle. The are the left and right coronary arteries, the left having a common trunk followed by the circumflex and anterior descending branches. Each of these give off side branches which bring blood and hence oxygen to the heart muscle. Coronary arteries fill during diastole.
There is a right coronary artery and the left coronary artery. In total there are four coronary arteries.