Yep it's a straightforward switch. Just get hold of an O2 PAYG sim card, mine was free on an offer but I think you now have to pay £10 but that will give you £10 credit. Opt in to the web bolt on which is £7.50 per month and your away. You just have to make sure you have £7.50 credit on your phone each month to pay for the bolt on.
If you top up by £10 you also get 300 free texts so as I hardly use mine as a phone I pay £10 per month for unlimited data and more than enough texts to cover my usage.
If you want to keep your old Orange number just ring up Orange and ask for your PAC code I think it is or PUK? code (can't remember now but just tell them you want to transfer your number). Then just give this code to O2 and they'll do the transfer. When they asked what model my phone was I just said I didn't know as I didn't want them to know I had an iPhone (for obvious reasons) but it didn't seem to matter.
Only downsides are you don't get edge speeds and you can't check your emails through the mail app you have to do it through Safari. I have a Yahoo address so this is simple enough for me. Everything still works when on WiFi though.
Hope this all helps. Like I say I'm much happier with O2 but if I were you I'd now wait and see what difference the new 2.0 software will make before deciding.