I personally think it is much easier to read Oedipus Rex in a book - rather than online or out of a text book. The best translation I've found so far is posted below. It is a more modern translation that was written for the purpose of film - so much of the poetic quality of the original play is lost. This means, however, that it is much easier to understand if you are reading it by yourself or aloud. It will help you get the main action of the story and the general personalities of the characters as well.
The other link is an easy-to-read online version. Again, the dialogue here looks pretty simple - and this too looks like it has lost much of its original poetic quality.