Friend of mine owned an oil company on Long Island. Just about every winter, there would be stories in the news papers down there of people dying of CO poisoning in their sleep.
He said the main problem is when they switched over from fuel oil to gas heat. With oil heat, the chimney flue gets a scale of soot. Over the years, that soot builds up, flakes off and falls down in the chimney. Normally, the high heat of the oil burner burns it off as it tries to build up in the chimney base. Not a problem unless the high heat of burning fuel oil stops.
When they convert over to gas, some gas companies will do a chimney cleaning. But, there are some private contractors who do the conversions for the gas company and some of them don't bother with the cleaning. With gas burning cooler than fuel oil, it can't burn off those soot flakes that build up and the chimney's will sometimes get clogged.
He said the owners should always request a chimney cleaning when converting from oil to gas heat, but too often, the gas contractors don't tell them.