Personally don't care for Onyx.
You can go the cheap route:
- Go to Safari menu
- Select Empty Cache
- Click on the Empty button.
- Go to Safari menu
- Select Preferences
- Select the Security tab
- Press on the Show Cookies button.
- Decide to remove all or remove certain cookies.
Or the cheap and simple route:
- Go to Safari menu
- Select Reset Safari (Note, this deletes everything: Cache, Cookies, History, etc.)