Rent a car.
krustytripper may well be right. There is no direct transportation from the Ottawa Airport to Montreal. The Greyhound station is in an odd location, on the fringes of downtown. From the airport it is two OC Transpo buses and a 5 minute walk. It would be easier to take a cab to the greyhound terminal, but that's not saving any money.
renting a car will be expensive as it will have to be a one way.
Maybe I should make the question clearer:
how can I get from the airport to downtown to get a bus and do it as cheaply as possible? A cab is probably what $20?
Aren't there airport shuttles to get to downtown?
would this do it?
take the oc transpo bus (#97, bells corner), to billings bridge station, get off and take the #1, direction ottawa-rockliffe, from there, get off at the bank and catherine stop and walk to the greyhound station, should take her a little over 30min.
30mins doesn't seem like too much
Maybe you should. Getting from Ottawa airport to downtown Ottawa is quite different than Ottawa airport to Montréal.
You can take the #97 bus from the airport to Ottawa Central Station, looks like 40 minutes or so. No idea what the bus fare is. There you can take any number of Greyhound buses to Montréal, looks like 2.5 hours or so.
DrGonzo... to answer your questions...
You have the bus figured out perfectly. It's anywhere between 30 - 40 minutes depending on the connection.
Fare is $3.30 (exact change on the bus), and it will drop you within a block of the station.
You are correct at $20 for a cab.
There are unfortunately NO shuttles downtown at this time.
I live in centretown and the cab fare is usually in the $30-$35 range. I'd say the estimate for taking the bus of ~40mins is about right, but that's from the time the bus leaves the airport (every 15 or 30 minutes depending on the time of day). The walk from the bus stop at Bank/Catherine to the bus station is about 200m. Buses to Montreal leave every hour, on the hour, from 0600h to 2300h.