Chiller is right, Omniweb uses a more recent WebKit than Safari, which allows site designers to choose whether buttons are rendered as you're seeing them, or as Safari does now. The former lets designers style form elements if they want, which isn't always a good thing. Safari will eventually become this way too.
Once the code in Omniweb is updated to a newer version of WebKit, you should be able to use a simple css file to specify that you want the Aqua style buttons.
More info here:
http://webkit.org/blog/?p=28