Scientific tools such as physiological analysis are fundamental to understand invasive species tolerances and responses to different environmental conditions. The Amazon fish Astronotus ocellatus, popularly known as oscar, was introduced worldwide and became invasive in many places. Specimens of A. ocellatus were abruptly submitted to salt stress of 14 and 20 ppt, for 3 and 8 hours to analyze its plasma osmolality and muscle wet weight change. Fish in 14 ppt presented no osmolality distress even after 8 hours. In 20 ppt a slight increase (about 10%) in plasma osmolality was observed for both times of exposure. Fish muscle slices submitted to hyper-osmotic saline displayed decreased weight after 75 minutes, and slices submitted to hypo-osmotic saline displayed increased weight after 45 minutes when compared to control. These results reinforce A. ocellatus’s eurihalinity. Despite the internal changes observed, the fish was able to regulate its medium and tolerate 14 ppt stress, but presented an intense osmotic challenge when facing salinities of 20 ppt.