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Let's say I have 2 connecting flights, first operated by airline A, second operated by airline B, on a ticket issued by airline C, booked on a booking site D. Who is responsible for rebooking and getting me to the final destination if the first flight is delayed and I miss the connection? A, B, C or D? If everyone claims it's not him, the other guys should do that, what document can I hold in front of one of them and say "No, you are responsible for rebooking, it's clearly written here"?

For the sake of argument let's assume I am at the connecting airport, airline A was a small regional with one flight per day and no presence (no ticketing desk, no representative) in the connecting airport and the online agency is already closed for that day.

I have read similar questions here, but none of the answers provided any sources for their opinion. If you are convinced A, B, C or D is responsible, could you please provide the basis for your claims, what documents / laws / conventions outline this responsibility in your opinion?

As is always the case for questions like this, there are a lot of different answers, and most of them are wrong - at least officially.

The rules for what happens in situations like this are created by the International Air Transport Association (IATA), and agreed to by all IATA member airlines (which includes basically every commercial airline in the world). The resolution that is specifically relevant in cases like you've described is IATA Resolution 735d (and to a lesser extent, 735e).

The simplified version of 735d is that when an "involuntary re-route" is required, it is the carrier that caused the re-route to be necessary that is responsible for arranging it (in IATA-speak, that is the "forwarding member"). In your example, that would be airline A.

In general, it is as simple as that. Airline A is 100% responsible for arranging the re-route. They could do that by putting you on a later flight from airline B, by putting you on one of their own flights to your destination, or even by booking you on a completely different airline.

Officially, airlines B, C and the booking site play absolutely no part in arranging the rerouting.

However, despite that being the rule, what frequently happens is that having been delivered late by airline A, you arrive at the gate for the flight on airline B and find you've missed it. In that case, airline B might decide that it's simply easier to look after re-routing you themselves. This will be especially true if they have another flight leaving later in the day with seats available on it. This is also not uncommon when airline A doesn't have any presence where you are at that stage (eg, you're now in the domestic terminal, and airline A only operates out of the international terminal).

The problem with going to airline B and asking to be "re-accommodated" is that they are not at all responsible for things like hotels or food vouchers that you may have otherwise have been provided by airline A due to them causing the delay. This is especially true in European Union countries where there are regulations around what needs to be provided - these need to be provided by Airline A, NOT by airline B!

In most cases it's the airline that would check you in for the flight segment that you missed. Example: We had a BOS->AMS, FCO->BOS itinerary booked on KLM. First segment flight was operated by Delta and delayed so that we would miss the KLM connection in New York. Delta (not KLM) rebooked us with Air France through Paris.

They also messed up our return flight on Al Italia, so we had trouble checking in for the return flight. That was fixed, again, by Al Italia, not KLM.

Think about it this way: each carrier has responsibility for you until that time they drop you off at then end of their segment and then the responsibility goes to the next carrier.

This does work smoother if the airlines are all part of a large alliance (Star Alliance, Sky Team, One World), but for most combo tickets, that's the case anyway.

I have been working in a travel agency for some years and I am pretty sure, if the flight is operated by two "unrelated" airlines, the distributor (which would be D in your case) is responsible of assuring your connection ... unless they manage to take off all responsibilities in their "Terms and Conditions".
I would say the best way then is to read the terms and conditions carefully; your answer should be there.
Anyway, they will always try to skip their responsibilities first, unless you insist.

Using your sample itinerary, Party C (British Airways) is the marketing carrier, not particularly the ticketing airline. You need to look at the ticket and see which airline is the ticketing carrier (in this case it could be BA or Finnair or both).

So first lets eliminate Party D (Orbitz), as they are simply agents for the airlines and not responsible for missed connections once the flight starts. Orbitz has responsibilities when it comes to changes prior to flying, but once you board, the responsibility to get you from A to C (via B) falls on the airlines.

Lets look at the ticket, was it truly a single ticket or was it an itinerary that had two tickets. Travel agents can create itineraries with multiple flights, with each flight ticketed separately or they can combine flights into a single ticket. This aspect is a major factor in determining responsibility. You need to look at the itinerary to see if there are two ticket numbers or just one.

If there are two ticket numbers, then you are hosed because Party B (American Airlines) and Party C are not responsible for delays in your arrival, you simply are counted as a no show. Likewise Party A (Finnair) has no responsibility because their contract is to fly you from A to B not C.

If it is a single ticket with a thru fare, then IATA's rules kick in (see Doc's answer)

For a customer relations point of view, if the delay was due to Finnair controllable issues, staffing, mechanical, etc, then Finnair should have stepped up and made sure you got rebooked.

notes....

I did not get into EU rules on delays and compensation as that is a whole other can of worms.

Ticket number is a long string of numbers starting with a three digit set, that is usually separated by a hyphen. The initial three digits designates the airline whose "ticket stock" was used ie the ticketing carrier. Finnair - 105, British Airways - 125, American Airlines - 001