From what I have read across all of the posts in this thread the negotiations have broken down over the issue of the master seniority list. BA Pilots want OS to be part of the list, BA don't.
From what I can see on the OS website, OS want to establish a meritocratic culture across the new company, including the pilots. This can't be done if they are part of the master seniority. This arrangement would give pilots from mainline priority over other pilots in OS for command positions, based on the likelihood that mainline pilots would have greater seniority.
OS are happy to take on pilots from mainline but they must compete for positions on equal terms with other applicants. What is wrong with that?