However, based on its etymology (combining "omni" meaning "all" and "dictionary" meaning "a book containing a list of words"), a reasonable pronunciation would be:
om-ni-DIC-shuh-nee
Here's a breakdown:
* om: like the "om" in "come"
* ni: like the "ni" in "night"
* DIC: like the "dic" in "dictionary"
* shuh: like the "shuh" in "shut"
* nee: like the "nee" in "knee"
Of course, depending on the context and the speaker's accent, the pronunciation could vary slightly.