From the OKC Zoo website;

On Saturday, July 17, admission to the Oklahoma City Zoo is FREE for everyone! In recognition of the 1/8 cent sales tax passed by the citizens of Oklahoma City on July 17, 1990, the Zoo is saying "Thank you!" to Oklahoma City with free admission. The Zoo will be open from 8 a.m. to 8 p.m. on Saturday, with exhibit buildings closing at 7:45 p.m. Sea Lion Show times for Saturday are 10:30 a.m., 1 p.m. and 3 p.m.