You want OmniWeb to launch automatically and open a specific page?
In System Preferences, click on Accounts, then select your account and click on Login Items. Click the + sign, and navigate to where you've put the OmniWeb application. Click the Add button. This will cause OmniWeb to launch whenever you log in.
To get OmniWeb to open a page at startup, you've got a couple of choices. One is to open the OmniWeb preferences, select the General preferences, click the radio button for Start Page and select the one that allows you to enter a URL (and enter the URL, obviously).
Another option would be to create a default workspace that has Auto-Save while browsing enabled. That will reopen any windows in that workspace when you start the browser. For details on workspaces, check out the tutorial at the OmniWeb product page, or the manual.