OSL - LHR - OSL in May 2015 with HBO fare:
NOK1300 ~ £123 pr leg
I have so far found the following rates at different times before a OSL-LHR/LHR-OSL flight
NOK640 (only with non-HBO)
NOK800 (only with non-HBO)
NOK900 (only with HBO)
NOK1300 (only with HBO)