No. Right to reimburesement only cover the cost of the ticket you are travelling on i.e. if you fly from LHR to CDG and then plan to continue to HKG, but upon reaching CDG you learn the HKG flight is cancelled and the next flight is 3 days out, you can say that the flight doesn't serve any purpose, ask to return to London and get reimbursed for your flight (and maybe get compensation and maybe not, depending on reason for cancelling the HKG flight).

You do have a right for re-routing at earliest possibility, and that does include rebooking on OAL. The staff will try to reroute you on partner airlines first, but you can insist on getting on, in this case, Cathay Pacific. You can't just buy a ticket, you have to go through the airline.