Gianluca Ros, 48, waited in the stairwell of the apartment building in the Norwegian capital, wearing the coat and a blond wig, as his ex-boyfriend returned from a holiday in the Canary Islands with his new boyfriend, Jørn Husby, on November 15th last year.
He then attacked Husby (40) with an axe, killing him, and then knifed down his 36-year-old ex, who was brought to hospital with life threatening injuries.
The 36-year-old told the court that he had been previously romantically involved with Ros, and that he had become fearful after he began making angry phone calls to him while he was on holiday with Husby.
"Since he was in disguise, I think he was planning the act," he said, according to Norway's public broadcaster NRK. "I saw a lady in a coat, and I thought that maybe she was lost. Suddenly, she attacked."
In court on Wednesday, Ros maintained that he did not consider himself to have split up from the 36-year-old, so his lover's decision to go on holiday with someone else had sent him into a rage.
"I wanted to take my own life, but a voice in my head said 'Why do I have to die alone?'" he told the court.
Ros' lawyer Stein Viken said that his client, who admits murder, was finding it hard to deal with the crime he had committed.
"He is generally not a criminal. He has ended up in a tragic situation that has a reason that will be revealed to the court. Knowing that he has taken a life, and almost another is eating away at him. It is of course a very hard time for him," Viken told Norway's TV2 broadcaster.
According to NRK, Ros has previously been convicted of attempted murder in the UK.
The 36-year-old is still suffering trauma from the attack.
"He dreads the trial. It will be very tough for him after what he has witnessed and experienced. He looks forward to being finished with the matter," Birgit Vinnes, the man's lawyer told TV2.
"Physically he is well, but the trauma he has experienced has made him sick, and he has still not fully returned to work," said Vinnes.