An official at the city's fire department confirmed the death toll of the clinic fire, which police are reportedly treating as possible arson.
"A total of 24 people died and four were injured. Whether their condition is critical or not is unclear," the official told AFP.
Japanese media said most of the victims were believed to have suffered from carbon monoxide poisoning, and may have been trapped inside the unit with the exit blocked.
Some patients saw a man placing a paper bag containing a flammable liquid next to a heater, which he then kicked over to ignite, the reports said.
The father of the doctor who runs the clinic said he did not yet know if his son was among the dead.
"I hope (my son) is safe and alive. That's all I want," he said in comments reported by several major newspapers.
The doctor had been having a hard time with many patients wanting certificates to apply for unemployment insurance, his father added.
Osaka in western Japan is a major economic hub and the country's second-biggest metropolis after the greater Tokyo region.
Deadly fires are unusual in Japan, which has strict building standards, and violent crime is rare.
One year ago, a man was charged with murder over a 2019 arson attack on a Kyoto animation studio that killed 36 people, the country's deadliest violent crime in decades.
The attack sent shockwaves through the anime industry and its fans in Japan and around the world.
And in recent months there has been a string of assaults involving fire and knives on and around the country's train network.
A 24-year-old man was arrested in Tokyo on Halloween for attempted murder after he allegedly stabbed a passenger and started a fire inside a moving train, wearing an outfit reminiscent of infamous comic villain the Joker.
That incident reportedly inspired a 69-year-old man to try to start a fire inside a running Shinkansen bullet train in the southwestern Kyushu region. No injuries were reported.