Once you're logged into OWA, over on the right side at the top of the page is a link to the Microsoft Outlook Web Access options page. The options page has a number of menu items on the left column. As you click through them, the contents on the right will change, much the way it does when you click through the menu items on the left in Outlook, i.e. "Mail", "Calendar", "Contacts", etc.
The account page has two columns. The left column displays some of your basic account information, including name, display name, and email address. You can also add your location address, and your various contact numbers.
The right column has a link to the Auto-Reply feature, previously known as the out of office assistant. Click this link to compose a message that will automatically reply to incoming emails, letting anyone who emails you know that you're on vacation.
You can also set two messages, one that sends to folks within your organization, and one that sends to external contacts. This could be handy if you want clients to know you're on vacation, but you want co-workers to know you're on vacation, and the keys to the server room or financial records are with the receptionist, for example.