"I was supposed to be on a plane that would have landed today," Oliver shared to Reddit.

"However, I managed to get an earlier flight yesterday; an earlier flight I decided not to tell my parents about."

Oliver explained: "They were still under the impression that I was going to come home later this afternoon and I let them believe that because I wanted to surprise them."

But when Oliver arrived home, it wasn't his mum opening the door to greet him – it was another woman.

"I locked eyes with the woman who answered the door... she looked young," said Oliver.

When he asked who she was, the woman candidly replied that she was his dad's girlfriend.

Enraged, Oliver questioned if this woman knew his dad was married. The woman replied, cavalierly, "yes", and told Oliver that his father was upstairs before walking off into the living room.

Oliver stormed upstairs to his dad to confront him, but his father was busy on a work call.

"I wanted him to tell me what was going on, starting with the random girl hanging out in our house while mum was gone," Oliver raged.

"My dad said he would explain everything after the Zoom meeting and then made me leave the room."

Downstairs, the random woman was watching Netflix in the lounge and munching on a bowl of cereal.

Confused and desperately wanting to find out answers, Oliver called his mum who reacted in an unexpected way.

She was seeing someone else too, Oliver's mum revealed to him. "She was actually with him at that moment," Oliver shared.

His mum told Oliver he was "never supposed to find out like this", but mum and dad are in fact in an open marriage.

"I was speechless," said Oliver, who didn't know what to do with himself in the moment. Later that day, when his father came into Oliver's room to explain the situation, his dad confirmed what he already heard from his mum.

"He did his best to convince me that seeing other people somehow made his marriage more meaningful," said Oliver.

"He said other people will always be other people whereas his wife will forever be the one." His father did caution, however, that from now on, Oliver should do no more unexpected visits.