There is nothing to worry about. You can login to your CIC account and check your application status. If it says approved, then your OWP is already mailed out and you should receive it soon in your mail (which you already have now). You will not get any other letter in the mail from the government. They notify you online about your OWP and you can see the message in your CIC account.For the ones who received the OWP email already, hace you received the letter by mail?
We received OWP on March 1 and I saw many people received by mail a couple of days before... Today is our 5th day and I'm starting to worry