An Oregon high school teacher is behind bars after an undercover sting operation led to authorities charging him with online sex crimes against children. 

Oscar Armando Ramirez, 36, allegedly had sexually explicit conversations with who he believed was a 13-year-old girl while working as a local high school teacher, according to the Portland Police Bureau. 

However, Ramirez was actually talking to an undercover police officer posing as a child in a sting operation targeting online predators, authorities said. 

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Oscar Armando Ramirez was arrested for allegedly having a sexually explicit conversation with an undercover police officer he believed was a 13-year-old girl in Portland, Oregon on June 6, 2025.  (The Portland Police Bureau)

“The chatting continued for four days and included lewd sexual conversations about acts that Ramirez wanted to do with her,” the Portland Police Bureau said in a statement. “In the conversations with [Bothell Police Detective Mike Garcia], Ramirez agreed to meet with her and made arrangements to meet when her parents would not be home.”

Ramirez allegedly revealed he was a high school teacher while the pair were communicating, prompting authorities to notify the Gresham-Barlow School District, which subsequently placed Ramirez on leave. 

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Oscar Armando Ramirez is charged with multiple felonies after police say he exchanged sexually explicit messages with an undercover police officer posing as a 13-year-old girl.  (iStock)

The Gresham-Barlow School District and Multnomah County District Attorney’s Office did not immediately respond to Fox News Digital’s request for comment. 

Last week, Ramirez was taken into custody outside his Portland home and charged with three counts of luring a minor and one count of online sexual corruption of a child. He is currently being held in the Multnomah County Detention Center.  

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Oscar Armando Ramirez was working as a high school teacher when police say he exchanged lewd messages with an undercover police officer posing as a minor.  (iStock)

The investigation did not reveal any crimes involving students at the high school or any other children, police said.  

Ramirez pleaded not guilty to the charges and is set to return to court on Tuesday, according to KOIN 6.

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Fox News Digital was unable to immediately locate an attorney for Ramirez.