In fact, he could choose to issue a smaller or larger sentence. Prosecutors had been seeking the maximum sentence of five to ten years.
Citing Cosby's frailty and blindness, Cosby's lawyers had asked for house arrest rather than prison time.
O'Neill's ruling on Cosby's status will not impact the ultimate sentence.
"I wanted 30 years, but I'm very happy to know Mr. Cosby will do time in prison", said Chelan Lasha, who gave tearful testimony in court about the time she says Cosby drugged and groped her in the 1980s.
"This has been a long journey to justice for all of the accusers.We're glad that judgment day has finally come for Mr. Cosby". That didn't change until the end of the two-day hearing.
AUGUST: Cosby hires new lawyers, including Tom Mesereau, the high-profile attorney who won an acquittal in Michael Jackson's child molestation case.
"This judge. has actually given a rather lenient sentence", Napolitano said. And he ordered Cosby to go to prison immediately.
3D gun advocate accused of sex with minor is jailed in US
Wilson, who lives in Austin, owns Defense Distributed , which sells blueprints for producing plastic firearms using 3D printers. The girl told authorities about their encounter last week but, police say, someone tipped Wilson off and he fled.
Cosby has been at the center of the #MeToo movement as dozens of women have come forward to accuse him of sexual assault.
Judge O'Neill said, according to TMZ: 'I'm not permitted to treat him any differently based on who he is or who he was'.
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A jury convicted Cosby of drugging and molesting a woman at his suburban Philadelphia home.
The actor's defence team had argued the state's sex offender law was too severe given Cosby's age and the fact that he is legally blind. A psychologist for the state testified that Cosby appears to have a mental disorder characterized by an uncontrollable urge to have sex with women without their consent. On Tuesday, O'Neill ruled that Cosby should be given the designation.
"Bill Cosby seeing the inside of a prison cell sends a strong message that predators - no matter who they are, from Hollywood to Wall Street to the Supreme Court - can no longer be protected at the expense of victims", she said.
As Vox points out, roughly 60 women have shared stories of alleged sexual assault, ranging from groping to forced kissing to rape, at Cosby's hands.
Throughout the nearly two-day sentencing, the prosecution reiterated Cosby's crimes and what they call his "predatory" nature: "He violated Ms. Constand's trust, which, unbeknownst to her, he spent time and energy cultivating with the specific intent of having sexual contact with her".