As previous responders have said – the two are a long way apart. To be honest using the tube is the quickest option (about 45 mins) – if you were to do this – you’d need to walk from St Pancras station to Kings Cross tube – and then have a number of options, the following are the easiest: (1) Northern Line from Kings Cross to London Bridge – change at London Bridge to get the Jubilee Line to North Greenwich (which is where the O2 is located) (2) Victoria Line from Kings Cross to Green Park – change at Green Park to get the Jubilee Line to North Greenwich. Of these the first would be my suggestion. If you’re insistent on not using the tube – then you could get a black cab – but will be expensive (£30-£40+ I would think). The only other option is bus or bus and boat – but these will take approx 1 hr 45 to 2 hours – options I have found at the Transport For London website (www.tfl.gov.uk) are: (1) – Kings Cross Station Stop: H Take the Route Bus 17 towards London Bridge Station. – London