Unfortunately it resulted in a pair of incomplete journey charge, because the time between the entry at Hackney and the exit at Willesden Green exceeded the maximum journey time, as the result of joining.
There are two concourses at Oxford Circus. One for exits 1-4 which are entry only, the other for exits 5-8 which are exit only, that meant I must exit at the concourse for exits 5-8 and enter at the other concourse.