It maintains sole custody of all votes, and is responsible for keeping the results confidential. Once the ballots have been tabulated, two senior accountants memorize every winner, and then prepare two briefcases with the envelopes used by presenters on the big night.
PwC says the briefcases are brought to the ceremony via "separate, secret routes." The two accountants stand backstage and hand envelopes to award presenters before they walk onstage.
It was a breakdown in the final stage of this process that led presenter Faye Dunaway to announce "La La Land" as the winner of Hollywood's most prestigious prize, only for "Moonlight" to be confirmed as the true champion moments later.
"I want to tell you what happened," presenter Warren Beatty said when it became clear that a mistake had been made. "I opened the envelope and it said 'Emma Stone, La La Land.' That's why I took such a long look at Faye."
PwC apologized to the casts of both films, as well as Beatty and Dunaway for the "error."
"The presenters had mistakenly been given the wrong category envelope and when discovered, was immediately corrected," the firm said in a statement. "We are currently investigating how this could have happened, and deeply regret that this occurred.
PwC has in the past portrayed its involvement in the Oscars as a symbol of its integrity.
"PwC's role in the Oscars balloting process represents the firm's lasting legacy of utmost accuracy and confidentiality," Martha Ruiz, who serves as one of the two envelope carriers, said in a statement issued before the 2015 ceremony.
Brian Cullinan, the managing partner for PwC's business in Southern California, carries the second briefcase.
On Sunday, his twitter feed was filled with photos taken backstage at the event -- including best supporting actress Viola Davis and best actress Emma Stone.
Later, after PwC issued its apology, all Oscar night tweets disappeared from Cullinan's account.
"We appreciate the grace with which the nominees, the Academy, ABC, and [host] Jimmy Kimmel handled the situation," PwC said in its statement.