We investigated the problem of oxygen-dislocations interaction and its influence on carrier concentration in GaN layers. We achieved the samples with various dislocation density. Then we checked carrier concentration by means of Hall measurements. The samples with a higher EPD were characterized by a higher electron concentration. We assumed that oxygen diffuses along the threading dislocation lines, acts as a shallow donor and influences unintentional doping. Hence, the more dislocations present in GaN layers, the higher carrier concentration.