A small hole in the wall (septum) that separates the right and left side of the heart usually is not any problem and doesn't require treatment.
If the hole is significant, there is what is medically termed shunting of blood from the left side to the right side chambers. If the hole is between the upper chambers (atria), oxygenated blood from the lungs will be shunted to the right atrium and recycled again to the lungs. Abnormal pressure can shunt blood from the right atrium to the left, and this would dilute oxygenated blood with blood that has not been oxygenated .
The hole can be between lower ventricles with the same effect. If there is right-to-left shunting the depleted and compromised oxygenated blood supply will cause what is termed hypoxia (low oxygen levels in heart tissue). Symptoms would include headaches, fatigue, shortness of breath.