If Europa has an oxygen atmosphere with a water mantle, is it possible that there could be life under the crust of Europa ice on the ocean where it is warmer? What could be the temperature under the ice?
No, nothing on Europa could possibly be photosynthesizing as we know it. Jupiter doesn't emit light, and what it reflects from the sun is not enough, plus there's no significant amount of carbon dioxide there.
However, the first life on Earth was not plants, it was chemotrophs. These bacteria gained their energy from the heat of hydrothermal vents, and if there's life on Europa, it would almost certainly take this form.
About the temperature: Europa's heat comes from friction due to tidal forces, so it comes from within. The water temperature increases as you go down, but the pressure is crushing down there and it's unknown whether life could form under those conditions.
The tenuous oxygen atmosphere comes from charged particles hitting it's cold surface and creating vapor. It does not indicate biological processes.