Hey, welcome to the club!
From the way you've described your story, it sounds like the OS is more likely to be in Activity than in Situation. However, you haven't given us enough information to be able to tell for certain.
When assigning throughlines to domains, I tend to think of it like this: which people are most concerned about the situation of the story, which are most concerned about what's going on, etc.? In your story, you have a concern of being "stuck in an alternate universe," which obviously belongs to one of your throughlines. So, ask questions. Who is concerned with being stuck in that alternate world, and who is concerned with returning to the normal world? Right now, it seems like the only one concerned with that would be your MC, which would put her throughline in Situation.
Meanwhile, fighting the ghost's attack on your MC involves at least two people who are primarily concerned with the outcome of that activity: your MC and the ghost. This fact alone makes it much more likely that your OS throughline is in Activity than in Situation. With that said, if this ghost is your story's Influence character (the character who forces your MC to change), then the fight between your MC and the ghost may actually represent the Relationship throughline of your story!
If your IC is someone else, of course, then obviously your RS throughline will not consist of fighting the ghost.
The nice thing about Dramatica (especially the software) is that you only need to know two of your throughlines' domains to find the other two, since your OS and RS will always be mirrors of each other, just as your IC and MC will be mirrors of each other. So if your MC throughline is in situation (which would be my guess), your OS will either be in Manipulation or Activity, depending on who the Influence Character in your story is and what role they play with regards to your MC.
If your MC is a change character, than start/stop has to do with her INNER arc. If she were a steadfast character, start/stop would pertain to her outer world/"the plot".
With that said, determining whether any character is start/stop can be quite tricky, and sometimes it's easier to let the software determine it for you (if you are using it). My inclination would be to say she's probably a STOP character, because of how you describe your story above: i.e. that your MC is being "chased" by a ghost of a person that she has hurt in her real life. In other words, she's not chasing the problem, the problem is chasing her! Thus, in order to solve that problem, she has to STOP running away.
Just a guess, though.