The babyfaced killer from Oklahoma who murdered an Upper East Side man 40 years his senior — after meeting him on a dating app — used a knife or a sharp ceramic object to mutilate his victim’s head, a Manhattan prosecutor alleged at his arraignment early Saturday.

The suspect, 24-year-old Alex Scott, had met 64-year-old Kenneth Savinski on Wednesday, the same day he allegedly killed him in a “vicious, ferocious, crime scene attack,” Assistant District Attorney Shira Arnow said.

The pair had gone back to the older man’s small apartment on East 83rd near Lexington Avenue after meeting on an unspecified app, police sources previously told The Post.

“We see the two of them on surveillance video, walking arm in arm,” the prosecutor said.

“The victim thought this would be a romantic liaison. The defendant thought this would be an opportunity to rob and attack,” the prosecutor said in court.

Inside the apartment, Scott attacked Savinski, stabbing him in the face and neck, severing his carotid artery and strangling him to death, the prosecutor said.

He then allegedly grabbed the victim’s watch, credit cards and cash.

“Two hours after we see him go into the victim’s apartment, he leaves,” the prosecutor said. “On surveillance video … we see him wearing the victim’s jacket, counting cash,” she said.

“Over the next day, he used the victim’s credit cards to buy himself food, to buy new clothing and cab rides.”

Then, on Friday morning, “he walked into the Midtown North Precinct with Savinski’s credit cards and ID on him and says he doesn’t remember the past few days, but he thinks he may have killed someone,” Arnow said.

Scott is still facing a trial in his home state on charges that he sexually assaulted a 6-year-old boy, Arnow said.

He had cut off an electronic monitoring bracelet in Oklahoma and traveled to New York before allegedly killing Savinski.

His court-appointed attorney argued he was violently ill in the precinct and may not have received the proper medical care.

She added that more facts in the case will likely come to light.

Scott was ordered held without bail.