A psychiatrist for the defense said yesterday that Mark O'Donnell's "cocaine-soaked brain" couldn't have formed the intent to kill 14-year-old Ebony Dorsey last year.

But O'Donnell's claims that he had a cocaine-induced blackout during the slaying didn't fly with the prosecution's psychiatrist, who said the only thing O'Donnell suffered from was a case of "selective amnesia."

Testimony in the five-day capital murder trial against O'Donnell, 48, concluded yesterday afternoon in Montgomery County Court. O'Donnell opted not to take the stand in his own defense.

Closing arguments are scheduled this morning before Judge William Furber.

After days of lurid testimony regarding the sexual habits and inclinations of O'Donnell and his former girlfriend, Danielle Cattie, who is Ebony's mother, witnesses yesterday testified about the effects cocaine may have had on O'Donnell at the time of the murder.

According to police, prosecutors and court testimony, O'Donnell is alleged to have stayed up all night on Dec. 6 and into the early morning hours of Dec. 7 ingesting either two or three bags of cocaine with Cattie at her Ambler home.

Sometime around 5:30 a.m., he left Cattie's to pick up Ebony, who was baby-sitting his 4-year-old daughter at his Plymouth Township apartment. When he walked in, O'Donnell claims to have seen Ebony near his daughter, whose diaper was off. O'Donnell said he believed that Ebony was molesting his daughter, though investigators said no evidence exists to support his claim.

Defense psychiatrist Dr. Kenneth J. Weiss said that incident, as O'Donnell perceived it through his "cocaine-intoxicated brain," could have been the trigger that sent him into a rage and left him out of contact with his "conscious thoughts, feelings and desires."

Weiss said the murder was hard to explain otherwise, as O'Donnell seemingly didn't have negative thoughts or particular problems with Ebony.

"There is a certain randomness and irrationality of this act," Weiss said. "The more random, unprovoked, unaccountable, purposeless, motiveless the act is, the more closely it is associated with poisoning of the brain."

Prosecution psychiatrist Timothy Michals said O'Donnell was able to play backgammon, drive a car and use a computer, all while high on cocaine that day. He said O'Donnell was able to remember that he killed the victim by strangling her with her own pajama pants. After the murder, O'Donnell hid Ebony's body to avoid being caught. All of that, Michals said, is evidence that O'Donnell knew what he did was wrong.

"I don't think being intoxicated would have prevented him from knowing he was killing this girl," Michals said.

Both psychiatrists had little time to study up on the case. According to court testimony, Weiss didn't interview O'Donnell until Wednesday, two days into the trial, and Michals wasn't contacted about the case until Friday.

Defense Attorney Thomas C. Egan III later told the Daily News that the psychiatrist he'd prepared for trial "opted out" last week after seeing the publicity surrounding the case. *