I tried to preempt any spraying by using Feliway all over. It seemed to do the trick as thankfully, he kept his pee to himself.
He's cautious and bold in his exploration. I loved seeing his tail held high. Every so often as he was checking things out, he would come back to me for a little reassuring head-butt - I love that he trusts me that I am comforting to him.
As you might imagine Oliver was not inclined to end his inside adventure. Once I got him back outside, he parked on the back step for at least an hour, waiting for me to let him back inside. When he finally left his post, he parked in the middle of the yard and just looked at me through the window.
After Oliver was back outside, the inside kitties were set free again. They smelled around the kitchen but were not freaked out by the new scent. Even Buddy seemed fine, which is great.
With this step having gone well, the next phase might involve Oliver meeting Theo and Nahum in the kitchen. I'll be sure to have on hand a noise maker and a water pistol to break things up if need be.