Wikipedia, certainly credible for the inventor of the internet, has this to say on the leader's connections to oil and big business:
Stephen Harper "briefly studied at the University of Toronto before travelling to Edmonton where he found employment in the oil and gas industry of Alberta as a computer programmer in his early twenties." 
Paul Martin: "Executive Assistant to Maurice Strong, President of Power Corporation of Canada Vice-President, Power Corporation Vice-President, Consolidated Bathurst Ltd. (A Power Co. subsidiary) Vice-President, Planning and Development for Power Corporation President and later Chief Executive Officer, CSL Group Inc. Chairman and Chief Executive Officer, Canada Steamship Lines Corporate Director for C.B. Pak Inc, Redpath Industries Ltd., Fednav Ltd., Manufacturers Life Insurance Co., Canadian Shipbuilding & Engineering Ltd. and Imasco Corp..." and much more.
And you know what Power Corporation means? Oil.
... plus a couple of other things, but who cares when your political vision is tunneled and your motivation embittered ...
Now, still using dubious internet techniques to prove a point,
google oil "Stephen Harper"
> 334,000 hits
google oil "Paul Martin"
> 528,000 hits
Then whip on over to The Ethics Comissioner's site and look up Paul Martin in the public registry search find these oil related records:
1995/09/27 - 2002/02/14
Cordex Petroleums Inc. (oil and gas exploration and production) (Alberta)
- 4.6% owned by The CSL Group Inc.
1994/02/24 - 1995/07/12
Argyll Energy Corporation
(inactive, formerly engaged in
oil and gas exploration and
production) - 10%
Cordex Petroleums Inc.(formerly Baca
Petroleum Corporation)(oil & gas
expl. and prod.) - 4.6%
Of course I'd like to see Stephen Harper's public registry entry when it comes available, but for now I think the line "backed by big oil" cannot be backed by a claim that because a man is from oil country, he is ergo oil backed.
update: good point, Mr Anonymous