An OSI stands for 'Other Service Information' and it is used to convey free-text information to specific carriers in a PNR. It's kind of like a remarks field, but more robust as the OSI text can be sent via messaging (and thus displayed) to other carriers in involved in the same PNR itinerary.

It doesn't have any other real function except to inform the agent reading the PNR, unless some robotics have been setup to read the OSI information and take an automated action.