There are a few reasons why a hiss or low level noise might appear in your mix after using Ozone:

Ozone's Multiband Dynamics settings may be configured to apply heavy expansion. Expansion boosts the volume of low-level signals, bringing them within audiable range, and in many cases it's a powerful tool for adding fullness to a mix. If a source recording has large amounts of low-level noise in it, however, this noise will also be increased under these presets.

To remove noise added through expansion, simply adjust the expander ratio to be 1.0 or greater. You should also make sure that you have not inadvertently enabled Ozone's dithering, which is a noise signal applied before bit-depth conversions. You can disable Ozone's dithering by selecting the "Loudness Maximizer" screen and setting the "Dither type" to "None."