First of all, welcome to Oberwiki!
If you haven't already, it's not a bad idea to create a username. This lets you create a user page about yourself and makes discussions easier to follow.
Oberwiki is a wiki site, which means anyone can add and edit information. You don't have to be a student, or live in the area, or be a graduate — just have an interest in Oberlin.
Creating a page
In a nutshell: search for the title of the page you want to create in the "Search" box on the left. If there's no page with that title already, you'll see a link to "create this page" (and then some search results).
What can you write about? Anything you want, that's related to Oberlin college or town in any way. Facts are great, but so are opinions, as long as you sign that they're yours (type four tildes: ~~~~).
Enter text in the large editing box, and put a summary of your edit in the Summary box below it. You can preview your edit with the "Show preview" button, and when you're done, hit the "Save page" button.
Once your new page has been created, it's visible to anyone — and editable by anyone.
When listing events, include the four-digit year in the date. That will avoid confusion next year, and so on.
For more information check out Help:Starting a new article.
At the top-right of (almost) every page, there's a black "Edit" link. (In some pages you'll see little  links to the right of section headings. Clicking on those links will let you edit just that section.)
Formatting: Oberwiki uses a special way of formatting text (making links, tables, lists, etc.). See Help:Editing for a description.
Images can be put in pages in a couple different ways. The easiest is to look at through the gallery of uploaded images, and then stick [[Image:<imagename.extension>]] in the page.
Oberwiki is run on the MediaWiki software, which is the same stuff Wikipedia uses. This means that much of the help stuff at Wikipedia applies here, too.