I've posted details on another site about these buses after several trips over the last couple years but it looks like it could be useful here too ...
Air France and KLM provide connecting bus services from Ottawa to Montreal Pierre Elliot Trudeau airport to / from Paris and Amsterdam.
First, when you book on AF / KLM that includes the Ottawa bus service, the itinerary will show that the origin is Ottawa Train Station (XDS) ... NOT YOW
These buses go to/from the Ottawa Railway Station on Tremblay Rd (not Fallowfield) and has the code XDS ... they don't go from Ottawa Airport or Ottawa Central Bus Station.
The buses are now operated by a company called Skyport International and usually brightly coloured on the sides saying Ottawa to Montreal Airport! (They used to be a plain white bus operated by Greyhound with a KLM or AF logo on the side.)
They have a KLM or Air France FLIGHT NUMBER and are quite rigid about departure times.
If you aren't on their list, you don't go (unless you have a boarding pass or itinerary with it mentioned). If you aren't there on time, you don't go. If you try to get on the wrong bus you may be rejected ... like an earlier bus or an "Air France" Flight when you have a "KLM" flight!
They depart from the 2nd lane at Ottawa Railway station and usually arrive about 10-15 mins before departure. Check in with the driver ... he has a list of names of expected passengers. There's a board inside the station doors with info about the buses and who to call in case of problems. Don't expect a lot of help since the call often goes to a call centre for Delta down in Detroit or Minneapolis who don't know much about the KLM/AF services!
No good asking the railway staff anything about these buses ... they know they come and go but not if they've come and gone! So, be on time!
Your bags go underneath and off you go ... about 2 hours later you're at Montreal ... you collect your bag and go check in / baggage drop.
To save time, you can check in online about 30 hours before the bus departure.
Beware the security lineup at Montreal is often very long. Go for coffee/food AFTER security.
Coming back, you will go through customs and immigration in Montreal and collect your bag. The new automated customs is usually a bit quicker.
Leave the new customs hall into the lobby, turn right and then right again ... follow to the end and outside. For a while there were signs directing people to the AF/KLM buses, but they've disappeared with all the new construction going on! If you ask airport staff, they'll shrug and point you outside or "down there"! Not very helpful.
Outside you'll see a road in front with buses and a car park opposite ... turn right and go to bus stop 7 or 8 (the bus will stop in front of one or the other) About 15 mins before the bus arrives, one of the handling agents for AF / KLM from Swissport (look at the lanyard his badge hangs on!) will wander around and check you off his list for the driver. He too isn't well organized considering they do this job a couple times a day every day of the year!
There are lots of buses through here ... and lots of people looking for their bus and you'll see other buses labeled Autocars Skyport (same company different service) and one labelled for a similar service to Quebec.
If you're late, and miss the bus, you'll have to find alternate transport.
It works ... but the distinct lack of info at Montreal end is frustrating.