You mention O2, are you travelling from the UK? Worth considering CMLink cmlink.com/uk - for example a one off payment of £11 to top-up and then purchase their 40G bundle will give you 4G of data in China that is not firewalled (the data is tunneled back to the UK) so all the socials continue to work if that is important to you.
Just make sure you disable autorenew so they don't spanner you for a 2nd month of charges.