Electrocardiogram recordings during opucal mapping experiments in heart tissue are commonly used tu monitor the health of the preparation and to obtain dominant frequencies during arrhythmic and defibrillatory studies. However the use of ECG reconstructed from optical mapping is seldom used and to date it has not been strictly validated. In this manuscript we present the first detailed validation and comparison of Optical Mapping ECG, or OM-ECG, with standard ECG recordings by calculating the electrostatic potential in space as a function of the voltage measured optically and describe the different approximations that can be used to obtain unipolar or bipolar ECG recordings. We found that in small/medium hearts, such as rabbits, leads that are aligned apex to base only require activation recording from one surface (anterior or posterior) for the OM-ECG to match the ECG while leads aligned left to right may require both an anterior and posterior optical mapping recording. The discrepancy between leads is due to symmetries in the ventricular activations. In the case of ischemic hearts where activations even-out more, the match between the OM-ECG and standard ECG may require only one surface recording for both left-right and base-apex leads. We believe that this methodology has two main and direct applications in the study of cardiac dynamics. The first is during studies of defibrillation where information after the shock may be crucial in the development of new strategies, OM-ECGs do not suffer the current artifacts of standard ECGs during shocks and can be calculated during the entire activation. We present examples in rabbit ventricles where even low amplitude pacing artifacts are captured by the ECG but do not appear in the OM-ECG. The second use of this technique is for reconstructions of intramural dynamics in larger hearts where differences between the ECG and OM-ECG obtained from anterior and posterior recordings can be used to derive the intramural activation.