The transfer between LHR and LGW (and vv) has no good options. I do this regularly, and consider it to be the low point of the journey between my end destinations (of which one is OSL, as yours).

I very reluctantly would recommend the National Express coach, not because it merits recommendation in its own right, but because it is the least of several evils. All tickets on the coach route are notionally for a specific departure. So if you trust BA to be on time, you can book in advance on line. At LHR T5 there is a set of ticket machines towards the south end of the arrivals hall where you can purchase your ticket. Just beware of the fact that the machines have a bug, at around the full hour they refuse to show the departures for the coming hours and you need to wait a few minutes for the software to behave. No joke, and this is important as you might lose your connection if you believe the machine and select a departure more than an hour away. P.t. the ticket is £19.50. With ticket in hand, walk out the southern exit, the coaches use platforms 14 and 15.

Now, the notional bit of the specific departure comes into play. You risk being refused entry to the coach if it is full. Again no joke. The staff just shrug their shoulders and ask you to get on the next one. On the other hand, if there is another coach departing before the one you're booked on, you might be able to get on it. Organised? Hardly. Competition? Nope, monopoly.

Once on the bus, the journey is usually uneventful, though the seats are not exactly Club Europe standard. The M25 is notorious for its traffic density, but Saturday morning you should not see more than 50 minutes to LGW.

Coming back is another piece of the same story, one ticket machine and usually a long queue in front of the manned counter. The lack of organisation applies here too, the specific departure is notional at best. If you've been to India, you'll recognise the behaviour in the line to get on the coach.

Another option is a black cab, they quote you £120. Travel time should be as the coach, but there is of course some more room, and you might have a good chat with the cabbie.

Or you can travel by tube to central London (about 50 minutes), change to the Circle or District line, get off at Victoria and get the train to LGW. More hassle, takes longer and is probably not worth it during the weekend as traffic on the roads should be easier.

I guess a fourth option is fly with someone-who-must-not-be-mentioned-in-this-forum to LGW from OSL, there is a morning departure. Way back BA also flew OSL-LGW, but you have to be old to remember those days. And there are other issues as well when you split your segments up between carriers.

Smile, listen to your iPod and turn your brain off for an hour when doing LHR-LGW-LHR, that's probably the best way.

Bon voyage!