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. 

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.

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