A legislative committee voted in favour of the audit Wednesday and directed the auditor to start it immediately.
The governing Liberals pulled the plug on the slots-at-racetracks program last year, which rocked the horse-racing industry.
The OLG has embarked on a plan to build new casinos in the province, which has also sparked controversy.
The audit would look into whether there are any secret or one-off deals regarding casino hosting payments to municipalities, as well as the cancellation of the $345-million-a-year share of slot machine revenues that went to the racetracks.
A legislative committee that voted in favour of the OLG probe has also directed the auditor to look into the Crown corporation's plans for gaming facilities, bingo halls, online gaming and lotteries.
OLG's president and CEO Rod Phillips said the agency will "support the AG's review in every way we can and we look forward to working with his office."
Auditor general Jim McCarter said work could start as early as this week, but that will delay another value-for-money audit that's already underway.
But he says it won't delay an audit of the cancellation of the Oakville gas plant during the 2011 election campaign, or a probe into the selloff of the Ontario Northland Transportation Commission.
McCarter is leaving his post May 1, but will be releasing on Monday his report into the cancellation of the Mississauga gas plant in 2010.
The Liberals voiced concerns at the committee about making a "rash" decision to change McCarter's workload.
But the Progressive Conservatives and New Democrats said time is of the essence because the government's move is killing the horse-racing industry.
Note to readers: This is a corrected story. A previous version said the OLG cancelled the slots-at-racetracks program. Although OLG oversaw the program, it was the Liberal government who made the decision to cancel the program.