Keswick Creek drain is 5 m wide between the airport and James Congdon Drive. This is plenty of room to fit a single O-Bahn track. Even if the drain needs to be lowered, it's still much cheaper than buying up neighbouring properties or building a tunnel. Because it sits in the drain, the O-Bahn is grade-separated, so it doesn't need to stop for anything. The proposed O-Bahn track is the blue line in this picture.
Buses come from the city on the red line, then enter the O-Bahn in the median on James Congdon Drive. The intersection with Sir Donald Bradman Drive would need to be modified to allow buses to turn onto James Congdon Drive.
If there are no buses on the track, buses can go straight onto the O-Bahn. Otherwise they will have to wait at traffic lights at the entrance to the track.
The drain is only 2.5 km long. Let's say the bus can average 50 km/h in the drain. It will probably be faster than that in reality because it's not stopping. That means it will take 3 minutes to travel the length of the drain. That's fast enough that we don't need passing loops, just have traffic lights at each end. Running these express buses at a ten minute frequency will be easy.
Having a single track bus lane is not a new idea. Here is an example from the USA where they have one bus lane in the median, used for buses travelling in both directions, and it's controlled by traffic lights.
Since it's in a drain, you might be thinking it will flood. That's why this needs to be an O-Bahn and not a normal road. An O-Bahn track can be raised off the bottom of the drain, so that water can flow beneath the track.
In recent years, there has been a lot of work to divert water from this drain to Brown Hill Creek, so it rarely fills up: https://bhkcstormwater.com.au/keswick-c ... iversions/ If it does fill up, so what? Just send the buses along Sir Donald Bradman Drive. That's what's great about the O-Bahn, it can use alternate routes. It's just a bunch of concrete beams, it's not going to hurt if it goes underwater every now and then.
This is so much cheaper than other public transport to the airport ideas and it's fast.