I'm some 10 months into an O2 online 18 month contract. The paper work
states that after 18 months, they will offer me a deal 'at least as good as
you can get in an O2 shop'.

What does this mean exactly. When the 18 months is up, won't I just simply
carry on getting the same minutes/per month as I'm currently getting, or
will O2 online force a different tariff on me?

Could I upgrade now, part way through a contract period at an O2 store? If
so, what is the in contract upgrade fee likely to be?

Si.