Once he realizes that the Null are ignoring him, Oscar will be using his not-on-the-hit-list status to his advantage for as long as possible. His main focus will be helping people out of the danger zones/evacuating people from those buildings and keeping tabs on everyone's location. If he becomes aware that any of his CR needs help, he will go to them right away. While he isn't an expert in first aid by any means he might be able to use bits of what he's learned if he finds someone in strife. He will escort anyone who's injured to safety. If he sees the Null closing in on any particular defenseless/injured person and no one else is there to cover them, Oscar will try to remove them from the situation; this might even involve him hatching a hasty plan to act as a diversion so the person can escape. If you want to arrange for something like this to happen, let me know!

Oscar will be interested in finding out whether there is a way to deactivate them since he'll believe that is their best solution for the time being. If he discovers their crowd-controlling weapons have no effect on him, then on top of knowing that their lasers don't affect him the same way they affect a living being, Oscar will want to help out in any plans involving disarming the Null as well.

Considering the Null's weapons and the swiftness of their attacks he probably won't want to risk driving a firetruck around to put out fires since he'll believe those efforts will be in vain, especially if he's alone, and will inadvertently endanger not just the Fire Brigade, but also the people who might decide to defend it. He might do it if the rest of the Fire Brigade are on board since he'll feel they need him, and that together they might have a slightly bigger chance of saving buildings, but he'll still advise against it. It looks like most of the Fire Brigade are going to be busy fighting the Null head-on, anyway, but I thought I'd mention it here just in case.