Actually just in general Barbara is a pretty decent subject for weight gain, whether she's Oracle or Batgirl.
You can see a scenario where she puts on some weight in college so she "temporarily" has to retire from being Batgirl. But her inner glutton keeps rearing its head, and she gets bigger and bigger, wider and wider, her belly oozing forwards, her butt cheeks swelling and sagging more and more behind her.
Eventually she acknowledges that she can't go back to being Batgirl because she's too fucking fat. And, hell, THAT'S when she becomes Oracle. That's a paralysis-free way to make her Oracle. She's become so fat and out of shape that she can't make it from one end of the block to the other without huffing and puffing, but that doesn't stop her from putting her master hacker and IT skills to use.
Not to mention sitting in front of the computer all day gives her plenty of time to snack...