More than 40 manhole covers have been adorned with hot-pink stickers, with the words "Bæsj, Tiss, Dopapir", or "Poo, Piss, Toilet paper", emblazoned on top in bold, eye-catching letters.
There is then a small script underneath explaining that nothing else should be put down the toilet, and that throwing food remains, coffee grounds and other unsuitable things down the toilet is like "inviting rats into your home".
"We chose pink because we thought it was the best way to get people's attention," Vivi Paulsen, a senior engineer at the city's Water and Sewerage Authority, told The Local. "It's to teach people what they should not throw into the toilet."
She said that the cost of the adverts, which were designed by Nucleus, an Oslo-based agency, was a fraction of what the city had to spend every year on cleaning out fat, old cotton buds and tampons from its sewerage system and water treatment plants.
The stickers were fixed on 40 manhole covers in high profile locations in shopping centres and central Oslo.