KA only flies the route four times a week and AFAIK this is the only OW service to/from KTM. KA101 KTM-HKG leaves late in the evening and arrives early in the morning in Hong Kong. The flight operates on Tu/We/Fr/Su.To get into OSL by March 15 you need to leave on Friday, March 13 at latest.

Given this schedule, I'd suggest you to fly as follows (I'm a fan of QF):
KA101 KTM-HKG 2250 0535+ 320
QF29 HKG-LHR 0800 1330 744
QF3589/BA768 LHR-OSL 1645 2000 320

Alternatively:
KA101 KTM-HKG 2250 0535+ 320
AY68 HKG-HEL 0945 1440 340
AY657 HEL-OSL 1645 1715 E90

Or:
KA101 KTM-HKG 2250 0535+ 320
CX257 HKG-LHR 0925 1500 343
BA768 LHR-OSL 1645 2000 320

It really comes down to price and preferences. You can choose among CX, AY or QF for the longer sector and others will be able to advise you on what is best in terms of quality etc. For me, it's QF.