There is no such thing as a OW ticket. The ticket had to have been issued by one carrier which presumably is a member of OW. Same with the HEL-LHR. There is an operating carrier. Could be others codesharing, but there's only one operating carrier.

As others note, if you are flying on an AY-operated flight using another carrier's codeshare for a single segment, it makes no sense and I'm surprised you were able to book it.

In any event, no matter what you think you hold, I would always confirm that the operating carrier shows you with positive confirmed space. Here, AY would have told you of the problem well in advance and it could hopefully have been repaired.