This change was inevitable...
One issue that I've seen is not all oneworld carriers use the same reservations system so it presents some technical issues. Within oneworld there's the following reservations systems being used:
Amadeus: QF, CX/KA, BA (moving to FLY which is based on Amadeus Altea), IB (not for departure control), JJ, RJ, QR, AB, AY, UL
Sabre: AA, LA
SITA: MH, S7
Not sure what JL uses, but apparently moving to Amadeus.
This can present quirks when you are through checked on separate PNRs from say QF to LA where QF uses Amadeus and LA uses Sabre. QF don't have control of the LA Sabre PNR, let alone view it fully and so rebooking due to delays can sometimes cause PNRs to become out of sync. A myriad of issues can occur when this happens. It also makes it difficult for a preceding carrier to provide protection in the event of delays.
In Amadeus, all oneworld carriers can see each others' PNRs, even if the viewing carrier has no flights in that PNR. For example in an all CX PNR, BA, QF etc. would be able to view it. This is why all oneworld Amadeus carrier bookings show up in QFF My Bookings. This is because Amadeus uses the same PNR record locator for the PNR, regardless of the carrier, and they automatically grant all oneworld carriers access to the PNR.
With Sabre, AA and LA would have their own copies of the PNR in the system and AA can't touch LA's copy, vice versa. The same occurs in SITA with MH and S7.
The main change from 01/06 onwards is that the minimum through check in and through tagging of baggage offered by the alliance becomes the following:
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1. Single PNR with record locator UVWXYZ, single ticket - through-checked/protected
Eg. QF ticket stock for SYD QF X/SIN BA X/LHR AY HEL
2. Single PNR with record locator ABCDEF, multiple tickets - through-checked/protected
Eg. QF ticket stock for SYD QF SIN
BA ticket stock for SIN BA LHR
AY ticket stock for LHR AY HEL
3. Multiple PNRs with record locators ABCDEF, UVWXYZ, single ticket in each PNR - no longer through-checked/protected.
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Going forward...
It's definitely possible to issue multiple tickets within the same PNR, but bookings made directly with an airline will not have this ability generally due to poor multi city booking engines offered on airline websites. Many multi city booking engines limit the fares offered to only fares published by that particular carrier. Most airlines also won't sell a fare published by another carrier, as fares generally have to be ticketed on the publishing carrier's ticket stock. QF can't issue a ticket for a BA fare that requires BA ticket stock for example.
If you want to issue multiple tickets within the same PNR, you'll generally need to have a travel agent do it. Most preferably, an agent that uses Amadeus given two thirds of oneworld uses Amadeus, as it'll be the same identical PNR viewable to the carriers as well as the agent.
It is NOT necessary to issue tickets for all connecting flights at the same time for them to live in the same PNR, so it's perfectly possible to buy the respective tickets during the respective sales if that's your thing. For example, the following is perfectly possible:
Itinerary: SYD QF X/BKK AY X/HEL AY LHR
SYD QF BKK - issued 31 MAR 2016
BKK AY HEL AY LHR - issued 20 APR 2016
There is a downside however...
If you are looking to dump sectors with hidden city ticketing... for example where the fare for LAX-X/PHX-LAS is cheaper than LAX-PHX (with your intended destination being PHX), you won't be able to do it easily anymore, if you want to be through-checked and the sector you want to dump is not the very last sector in the PNR.
By having all your sectors in the same PNR, if one of the carriers cancels your itinerary due to no show, it'll generally cancel ALL future sectors in the same PNR, even if they were issued on separate tickets. Example:
Itinerary: SYD QF X/BKK AY X/HEL AY LHR // LHR BA X/BKK QF SYD
Assumption: AY fare for BKK-HEL-LHR is cheaper than BKK-HEL
Tickets:
QF ticket stock: SYD QF BKK // BKK QF SYD
AY ticket stock: BKK AY X/HEL AY LHR
BA ticket stock: LHR BA BKK
If AY cancels due to no show on HEL-LHR, then AY is able to cancel all remaining sectors, that being LHR BA X/BKK QF SYD.