"He has denied global warming as a scientific reality," Mulcair said at a news conference in London, Ont.
"And as one of the chiefs in British Columbia who was the object of his invective said, his attacks on First Nations go beyond mere paternalism. They're tinted by racism.
"Joe Oliver is an embarrassing nomination as finance minister for a G7 country," Mulcair said, adding that Oliver's "information on global warming has consistently proven to be fanciful."
Oliver is leaving the natural resources portfolio to take over the finance ministry from Jim Flaherty. Flaherty resigned from cabinet yesterday, indicating he will eventually take a job in the private sector. Flaherty will remain an MP for the time being, his spokesman told CBC News.
Oliver had suggested he would speak to reporters after his swearing-in, but instead said only a few words as he climbed into a waiting car.
Mulcair said he found the "secret swearing-in ceremony" amusing.
"It's like a cult that he's just become a member of. If they're proud of Joe Oliver as minister of finance, why in heaven's name wasn't the media allowed to attend the ceremony? I do think it's a sign that they're not even sure of their own appointment, frankly," Mulcair said.
Oliver has a law degree from McGill and a Harvard MBA. He worked in the investment banking industry and served as executive director of the Ontario Securities Commission.
Rickford, the MP for Kenora, Ont., moved to natural resources from minister of state for science and technology. He has also been parliamentary secretary to the minister of aboriginal affairs.
Oliver often clashed with environmental groups and others who opposed expanded oil and gas development, famously calling them radicals.
Mulcair said he wishes Rickford well.
"Let's hope that Mr. Rickford can correct some of the things that Joe Oliver's been getting wrong," he said.
Prime Minister Stephen Harper was at Rideau Hall for the swearing-in, for which no press release was issued. Harper was hidden from reporters and photographers as he entered and exited Rideau Hall.
Also on HuffPost