I have used OE for longer than I care to remember and have never seen a Send As function.
In OE you have a default account that would normally be used to send emails. To temporarily change the default email account/sender (Send As a different account), when you are composing/writing the email check the Top of the email for the From: window, it will show the default email account address, to the right of that you will see a small down arrow, clicking on that will display other accounts that you could use.
Try to do what it is you are calling Send As and jot down what it is that you are doing because there isn't enough information to identify your problem.
