UA planetary scientist Richard Greenberg calculates that the subsurface ocean on Jupiter's moon Europa could have an oxygenated layer more than 300 m thick, with oxygen levels being generated that are high enough now to support 3 million tons of macrofauna, "assuming respiration rates similar to terrestrial marine organisms."
The study was published recently in the journal Astrobiology. [below, animation from NASA on a possible mission to Europa's ocean]
Richard Greenberg, "Transport Rates of Radiolytic Substances into Europa's Ocean: Implications for the Potential Origin and Maintenance of Life," Astrobiology, April 2010, 10(3): 275-283, doi:10.1089/ast.2009.0386