I have no experience with this feature, but here is what I did to set it up:
The speech preference in OmniWeb is in the General preferences (Speech: Navigate web pages using speech recognition). When I select that, the system's Speech widget appears:
If you press Esc, you should be able to talk and have OmniWeb do its business (for a list of commands, click the little triangle at the base of the Speech widget and select "Open Speech Commands Window"):
You will also need to visit System Preferences>Speech to change the settings to better suit you. FWIW, I can't get it to work as it doesn't like my accent.