When you've clicked through to the entry for the word you're looking for, the new window will offer a series of choices to limit the information you're seeing or to provide more information.
The central screen offers a basic definition or series of definitions (click on the image to view a larger version):
As you can see, an OED Online entry offers much more information than simply the word's definition. By moving your mouse over the highlighted links in an entry, you can see a word's pronunciation:
You can see a word's etymology:
And you can see quotations that place the word in historical context.
The sidebar to the right of an entry offers more information to place the word in context, providing you with a list of all the words that appear before and after your search term in the OED Online:
Clicking "Date" at the top of the sidebar will bring up a list of other words that originate around the same time as your search term:
The OED Online doesn't just list words that are currently in usage and of English origin: it aims to be a comprehensive chronicle of the English language. And so it also includes words that are obsolete, or non-naturalized or non-Anglicized. How will you know when you're looking at obsolete or non-Anglicized words? A couple of examples:
In the entry above, the dagger to the left of the headword indicates that this is an obsolete usage: no contemporary writer would use "library" as a synonym for "scribe," but this was an accepted usage in the past. Frequently, the OED Online will list obsolete usages alongside current ones, also marking them with a dagger, as in the entry below (the definition highlighted in yellow - click on image for larger view):
An OED Online entry will also provide you with information about the history of an entry (click on image to view a larger version):
In the case of the entry immediately above, the most recent updates made were in December 2012, and the previous version appeared in the 1989 print edition. You can click on "Previous version" to see how an entry appeared in a previous edition, or you can click on "About this entry" to learn more about it, such as when it first appeared in the OED and which other entries link to it.