Originally Posted by xliioper

For Europe, if you are booking far enough out to qualify for the cheaper roundtrip fares with lengthier advance purchase requirements, it's almost always going to be significantly cheaper to book as a roundtrip. For Asia fares, the differences are generally not that significant and there may be cases where 2 one-ways are cheaper.

TK roundtrip J fares on SFO-IST start at $4174 roundtrip for cheapest AP fares, while one-way J fares start at $3312 for SFO-IST and $3612 for IST-SFO. UA J fares start at $4738 roundtrip for SFO-IST and $6153 one-way for SFO-IST and $1985 one-way for IST-SFO. You can see it's pretty clear which carrier dominates point-of-sale in each direction from the one-way pricing.

In some limited cases, it might potentially be cheaper to book TK one-way on SFO-IST, and UA one-way for return on IST-SFO (but it's still going to cost more than cheapest possible RT fares on either TK or UA booked on a single ticket as can be seen from their RT fare pricing).

If you are making multiple trips, you can play roundtrip fare nesting tricks with roundtrips ex-IST nested in roundtrips ex-SFO for even cheaper total pricing.