Oklahoma City has one of the best independent iron distance triathlons in the country, the REDMAN triathlon. The Redman triathlon weekend has four different races - full iron and half iron distance, and full and half aquabike.
The race venue is at Lake Hefner in northwest Oklahoma City. The lake is large enough for a 1.2 mile swim loop, although close to shore, you can stand for part of the loop. The water can be a little muddy, especially if it rains.