I am looking at a J class flight to Australia next year. The price from LHR is more than double the price from OSL.

The routing from LHR is LHR - HKG - MEL - HKG - LHR
The routing from OSL is OSL - LHR - HKG - MEL - HKG - LHR - OSL

The flights from LHR to MEL on BA and CX are identical. At less than half price I will obviously reposition to OSL to take the flight. My question, however, is on the return leg, is there any issue if I just book bags to LHR and get off there and do not take the last leg back to OSL. Obviously from a ticket perspective none, however, could there be consequences with regards miles being credited for the flights (would credit the miles to AA)?

Any advice welcome and appreciated.

Search for "hidden city ticketing" for plenty of existing threads on this.

YMMV on whether you can have your bags checked to just LHR.

You might have a problem. Well, two, actually.

(1) What you described, mainly skipping the last leg on the itinerary, is known as "Hidden City" or "Throwaway Ticketing". It's a common practise for many experienced travellers but it's frown upon by airlines. BA was actually making waves about this practise recently after, apparently, many UK travellers took advantage of relatively cheap business class tickets from Germany to HK and skipped the last leg back to Germany. It was said that BA was threatening to shut down Executive Club accounts and what not, but, I've never seen any official note on their site and never met anyone that an action was taken against him.

Since it's your first time doing it - I won't worry too much.

(2) If you'd like to collect your bags in LHR you need to have a really good reason way. For the check-in agent in Australia LHR is just a transit point, so, your bags need to go to OSL. The easiest way to do it is by creating a stopover at LHR - if the ticket rules permit - and scheduling the last flight a few days after. This way your bags will be tagged to LHR and your points will show up on your account before the last flight you wish to skip. Or, at the very least, try to put in an overnight layover at LHR so at least you'll have a good excuse to tag your bags to LHR instead of OSL.

Thanks for the responses

Lots of scary stories.

Ive ha plans change on me, and I dont take flights. BFD. Happens.

If you use a good travel agent, they can simply cancel the LHR-OSL leg. Not rebook, just leave an open ticket in the record. This way the carrier on LHR-OSL is not looking for you.

The problem is you cannot 'book bags to OSL'. Bags will be tagged to the ticketed destination.

I do like the suggestion of creating a >24hr stopover at LHR to force LHR as a bag destination. All depends on the underlying rule for SYD-OSL, Some fares allow a stopover; some do not and you bounce to a higher fare; some add a fee GBP100, 200 or whatever

I think if you do it on occasion, you should be ok. If you do it habitually, they will know. I don't think they can send you a bill and charge you for the difference in fare, BUT the airline can ban you from flying with them in the future.

They could also withhold the miles that would be earned. At the very least you wouldn't get the miles from LHR-OSL.

The ticket may also have a "no-show fee" - I have one (where I am intending to use all sectors, btw) where the no-show fee is US$500

That's actually not a major problem. First, you can avoid a No-Show by contacting the airline and cancelling your seat on the flight. You basically asking to change the ticket but without selecting a new date for the flight - and thus, usually, you won't be charged the change fee yet. Second, even in case of a No-Show the airline would have a problem to collect the fee as you can always refuse to pay - and I highly doubt any airline would want to go to court over this fee (given a regular customer who bought a business class international ticket and missed one flight).

IMO the only worry is the FF account because it's an easy target for the airline in case it wants to 'punish' the passenger.