Yes energy can be released without O2 but it will be lesser as compared to that produced in the presence of oxygen. Ex-->fermentation

Yes energy can be released in absence of oxygen .Anaerobic respiration include oxidation of respiratory substrates in absence of oxygen is termed as anaerobic respiration it involve incomplete breakdown of respiratory substrates in which the end product such as ethanol , lactic acid of form and Co2 is released as in case of aerobic respiration the initial breakdown of glucose includes the presence of oxygen for the break down in absence of oxygen results in the production of Ethanol lactic acid anaerobic respiration occurs in certain microorganisms such as bacteria and yeast these micro organisms get energy in absence of oxygen by anaerobic respiration