I'm going nuts here. I know there's a simple switch somewhere in the Omnisphere interface to reverse a sound (play it backwards). Someone PLEASE enlighten me. My brain is frustrated!
There is no dedicated reverse function in Omnisphere. But you can find reverse versions of some of the soundsources in the sound browser.
Just type the word reverse into the searchfield to find them.
As an alternative you can use the envelopes to simulate reverse sounds. There are a few reverse effect envelopes on the presets, or you can freeze/ render the sound and reverse the audio it in the host.