Court filings say Purdue paid McKinsey more than $93 million over 15 years for a number of products.
The Sacklers would have contributed up to $6 billion and given up ownership of the company but retained billions more.
A bankruptcy judge must still approve the deal.
While OxyContin's maker faces penalties and forfeitures totaling more than $8 billion, no individuals admitted to crimes.
Advocates are upset that the guilty plea applies only to the company and not executives or members of the Sackler family who own it.
The officials say Purdue will plead guilty to three counts, including conspiracy to defraud the United States and violating federal anti-kickback laws.
OxyContin maker Purdue Pharma is rolling out an advertising campaign to let people know they can file claims against the company over the toll of its opioids.
The nation's three largest drug distributors and a drugmaker have reached a tentative deal to settle a lawsuit related to the nation's opioid crisis.
UPDATE: The proposed settlement with OxyContin maker Purdue Pharma would raise money to clean up the opioid mess by selling more opioids.
Public officials in Akron, Ohio, say no amount of money will restore the families and institutions that have been upended by the opioid crisis.