No catch that I've come across, and I've been with o2 online for two years now.
I got an 18 month contract last time, and got 50% extra texts (750, rather than 500) and minutes (750 X network, off peak, instead of 500) for £25 a month.
A while ago I changed my tariff, which they let you do for free, apparently you can change it as many times as you like, as long as you wait at least 30 days between each change. I then changed back when I renewed my contract - I'm indecisive!
When I first got my phone, I had a few problems with getting media messages so I took it to an o2 shop and they were really helpful there, which I wasn't sure they would be as I'm online customer.
In my experience, they are no catches with the online deals! Hope this helps!
(The only things to be wary of it that the X network, peak rate calls can be quite expensive - but that might be the same for the high street tariffs?)