Ingham County Prosecutor Carol Siemon requested a special prosecuting attorney from Michigan Attorney General Dana Nessel's office on Feb 22. Nessel's office retained the prosecution of the case. 

As a detective, O'Brien was involved in a 2014 Title IX investigation into now convicted ex-MSU doctor Larry Nassar's sexual abuse. O’Brien was assigned to survivor Amanda Thomashow’s case but cleared Nassar of Thomashow's report of sexual abuse.

Ingham County Assistant Prosecutor Debra Rousseau said in a 2018 report that O’Brien failed to “consult additional experts, despite being asked to do so.”

After over a year on paid administrative leave, O'Brien was demoted from the assistant chief at MSUPD to captain. She returned to duty on Sept. 8, 2020, as a Captain for Special Projects after being on hiatus since April 2019.

According to documents, MSP Trooper Andrew Golden pulled up behind O'Brien's vehicle to initiate a car assist, following Ingham County Central Dispatch airing a Be-On-The-Lookout for her car. Golden then arrested O'Brien for an OWI, after further investigation.

O'Brien refused to take a chemical blood test. A search warrant was authorized, and O'Brien was transported to Sparrow Hospital where a phlebotomist obtained her blood sample.

On Feb. 19, lab results revealed O'Brien's blood content was 0.25%, more than three times the legal limit for driving, which is 0.08% in Michigan.

As O'Brien was transported, she was "extremely belligerent" and said "f⁠—⁠—⁠— you" and told Golden she will sue him.

"I will own up to the fact whatever I blew was over the legal limit," she said while in the patrol car. "I don't give a s⁠—⁠—⁠— if I get fired. I don't give a s⁠—⁠—⁠— if I get arrested for drunk driving."

O'Brien was later transported to Ingham County Jail where she was lodged for the offense.

As she was transported, O'Brien asked, "What would have happened if I blew and I was under, I could've went home? I would've been over anyways."

Following the arrest, Sgt. Marle Seymour of Ingham County Sheriff's office determined O'Brien had her department issues firearm in a duty bag located behind the driver seat of her vehicle. The firearm was released to MSUPD Sgt. Michael Aguilera.

According to the Lansing State Journal, O'Brien's attorney, Patrick O'Keefe, said, "You need to understand, addiction and alcoholism is something that affects everyone."

However, O'Keefe declined to specify if O'Brien was facing either.

"She obviously had a bad night," O'Keefe told LSJ. "She's been working very hard in her recovery...I believe she's back to being the same officer I knew when I was a prosecutor, prosecuting sexual assault cases with her side-by-side." 

In an email statement, MSUPD Captain Chris Rozman said, "We are aware of the charges brought against Ms. O'Brien by the Michigan Attorney General. It would be inappropriate for the MSU Police Department to comment on the pending criminal charges being brought by prosecutors."

Following her arrest by MSP, O'Brien was placed on unpaid leave with no assigned work duties. O'Brien resigned on May 1.

O'Brien's arraignment is set for June 7 at 8:30 a.m. in Ingham County 55th District Court.