In Omega theme you can create a page and set it to be your 404 page. To do that go to Omega->General and set the Page Link option to the page that you want.That's all!
In Omega theme it is easy to create a one page template version. All you have to do is create sections and assign a unique id to each.
After creating the sections add a menu using the section id's in the menu elements.
Take a look at the video on how to create a one page template :