Can somebody help out on how to look for planes/routes which has double/upper Deck ? Does it have be to first class or can Business Class is also have access/allowed to Upper Deck ? I have never flown the plane with upper deck and wanted to plan something special for Nov 2022 or Dec 2022. Do we have any routes from US also with upper deck in one world airlines or these only outside US routes ?

Getting to be slim pickings. I think British, Qatar and Qantas are about your only bets at this point. And all three of those are A380s.

Qantas A380s actually have coach in the rear of the upper deck. Ive done it. Tucked away little cabin that is not bad.

I WAS thinking the same about QF but looked at a random A380 seat map LAX-SYD and all no more econ on the upper deck - all are PE. Of course, not all A/C may have been changed out...

Of Oneworld Airlines, only 4 operate double deck aircraft (British Airways, Qantas, Malaysia, Qatar), all of which are A380s. Malaysia will be selling/retiring theirs soon. Qatar and British are currently flying them, and Qantas will start flying theirs again soon. With COVID restrictions and demand changing, it's hard to tell where they will be flying later this year. I know Qantas will start flying LAX to SYD in March on the A380, but it remains to be seen for how long. Pure guess, but, if the travel restrictions relax as expected, I am sure there will be a large demand this year, and quite possibly Qantas will keep this on that route, as well as on DFW to SYD. The advantage of Qantas is that they also refitted their A380s. Qatar and British Airways still have hard products that a quite inferior to the rest of their fleet (BA and Qatar have suites on many of their 777s).

But didn't that very same retrofit remove those few Econ seats on the upper desk, making the upper desk all Business and Premium Economy?

Yes. I guess I should stipulate that, just looking at hard product, British Airways or Qatar would be the best choice for economy travel as they have economy in the upper deck, and Qantas for Premium Economy and Business. Qatar has their First class on the upper deck, so if you are looking to shell out that kind of Points/Cash, it's also a good option.

Go to ita matrix.
https://matrix.itasoftware.com/

Click on advanced controls under to the to/from airports. Its not listed in the examples, you can insert an Extension code using “AIRCRAFT t:xxx”, where xxx represents the IATA code of each aircraft. For example, if we’re looking to fly on the A380, the full input would be “AIRCRAFT t:380”. (You can specify ONE world flights only as well), so you text would be:

AIRCRAFT t:380; alliance oneworld

If you already know it is a 380 or 747 flying the route, you can check seating on:
https://www.seatguru.com/

Do bear in mind though, that in these times aircraft type frequently change at short notice. Good luck, the upper deck of a 747 was my absolutely favourite place in the sky.

Upper deck is now limited to B747 & A380. But very few airlines are now flying the B747
There is a very good (new) seat map site written by BA forum FTer --> https://www.aerolopa.com/ and Introducing aeroLOPA: a new portfolio of detailed BA cabin layout plans Seatguru and other like sites are out of date (and have been for years)

A380's in the air now-->https://flightaware.com/live/aircrafttype/A388

QF are flying the A380 now. 1 will be flying LAX-SYD in a few hours time. 1 of the QF A380's flying now has no been refitted.
https://flightaware.com/live/flight/QFA11
https://flightaware.com/live/flight/QFA12

ExecTrav AU 01 Feb 2022 EK (QF codeshare)--->Superjumbo soars again as Emirates brings the A380 back to Melbourne
ExecTrav AU 18 Jan 2022 QR--->Will Qatar Airways’ Airbus A380 return to Sydney?
ExecTrav AU 11 Jan 2022 QF--->Qantas Airbus A380s return to the skies

What other options are there in the modern jet era? I guess OP could have rolled the clock back 85 years and taken a zeppelin....

FWIW I�d specifically look for a 747. The UD provides that intimate feeling you are probably looking for. A380 feels more like 2 planes stacked on top of each other.

Don't think any oneworld airlines operate 747s these days (other than CX, QR and QF Freight divisions).
BA and QF retired their 747-400 fleets in 2020, and CX a few years earlier.

Only 747-8 operators are LH and KE.

As above their will be a short time where you might be able to get upper deck in Economy on QF aircraft before they complete the refit to a J/PE only upper deck.