I haven't checked, but I've a feeling that eucalyptus oil is very toxic to cats, so use with caution. Lavender should be fine, but it doesn't clear the airways as well. Just putting water in an aromatherapy lamp (the bulb-shaped things you put a tea-light inside; they have a saucer-like bit at the top where you put the water and a drop of oil) will humidify the air and help puss. They don't create steam, just atmospheric dampness. Dry air from central heating would exacerbate Swampy's breathing problems.
Unfortunately, the best oils for clearing the nasal passage contain menthol, which I'm sure is toxic to cats.