And here we go again - don't you ever listen?
OSC has absolutely nothing to do with the ship being more stable for helicopters. OSC is about coordinating the rescue operation. MRCC will assign OSC to a ship that is best suited. They will evaluate things such as:
* Previous experience
* Ability to set staff to the side just to coordinate the rescue operation
* Ability to communicate with all other ships and helicopters, on multiple different channels at the same time
* Personal knowledge of people involved
* Good place on board the ship where the OSC staff can work, with charts, radios, radars and so on, without disturbing the other work happening on the bridge.
Some ships have a dedicated workspace for OSC work. I'm sure MRCC had all these things in mind when then allocated the OSC. I'm also sure that they didn't care at all about the helicopter deck, since that has absolutely nothing to do with OSC. OSC is about leading, coordinating and communicating.
I've seen situation where the OSC is not even taking direct part in the rescue work. Basically all on the bridge were busy acting as OSC, so they stayed out of the way and focused on coordinating the work of the other rescue crafts involved.
And again, the initial coordinating role that Silja Europa took on themselves is separate from the OSC role that they later were assigned by MRCC. The major difference is - during the initial coordinating role they do their work without a formal mandate from MRCC - instead the mandate come from SOLAS/IMO regulations, and an agreement with the ships around.
At a later stage MRCC, when MRCC was fully involved and had taken over the overall accountability for the rescue operation, they assigned Silja Europa the OSC role.