Before that order, most of the case's disputes had swung in favor of prosecutors, including O'Neill's decision past year to allow them to present at trial Cosby's own damaging, decade-old deposition testimony in which he acknowledged offering drugs to women he wanted to seduce.
Since then, however, dozens of women have come forward with allegations of sexual assault, and O'Neill ruled that prosecutors are not bound to whatever verbal agreements their predecessors may have made.
Cosby, 79, is charged with indecent aggravated assault for allegedly drugging and sexually assaulting former Temple University employee Andrea Constand, now 43, at his Elkins Park, Pennsylvania, mansion in January 2004. O'Neill had earlier ruled that jurors could hear Cosby's damaging testimony from Constand's 2005 sexual battery lawsuit. The star of iconic shows such a the Cosby Show has claimed that the encounter was consensual.
"Branded variously a monster, sociopath, and sexual predator, everyone from the current District Attorney of Montgomery County to President Obama has publicly weighed in on the allegations against Mr. Cosby".
The additional accuser who can testify worked for one of Cosby's agents and had known the entertainer for six years when he invited her to lunch at his bungalow at the Bel Air Hotel to discuss her acting ambitions.
But the New York Times said that the judge only agreed to allow the testimony of one woman, identified simply as "Prior Alleged Victim Six", in a one-page decision.
The woman said she turned down the wine he repeatedly offered her but that, at his insistence, took a pill that Cosby told her would help her relax, according to court documents.
Prosecutors argued the testimony is relevant "to establish a common plan, scheme or design" for the jury and "to establish that an individual, who over the course of decades intentionally intoxicated women in a signature fashion and then sexually assaulted them while they were incapacitated, could not have been mistaken about whether or not Ms. Constand was conscious enough to consent to the sexual abuse".
The ruling is one of two key pretrial issues in the case. His lawyers have said the accusers were being "paraded" before the media by celebrity lawyer Gloria Allred before their accounts are even vetted by police.
If convicted of the charges at trial, Cosby faces a possible maximum sentence of 15 to 30 years in prison.
Bill Cosby's trial is set to begin in June. He is free on $100,000 cash bail awaiting his trial.