I was surprised to see this included here, simply because most authorities on modern aromatherapy counsel quite strongly against the use of this oil, as it's potentially quite toxic.
So - warnings first. Hyssop (and remember, we're talking about the oil here, not the herb, which is quite a different matter) contains a ketone called pino-camphone. This is now believed to be highly dangerous to anyone who suffers from epilepsy or any sort of seizures. It should also be avoided completely by anyone with high blood pressure or who suffers from migraine. Some old texts will actually tell you to use Hyssop oil as a remedy for these conditions. Don't!
Having said that, it does still have some potential uses in modern aromatherapy.
Its main use is probably in treatment for chronic lung conditions (look away now if you're eating your lunch/tea/ dinner), especially where there is thick mucus. The oil (inhaled or used in an aromatherapy massage) helps to liquify this, enabling it to be, erm, coughed up rather more easily.
Hyssop may also be worth trying (diluted at 1.5% in a carrier oil) for anyone with especially nasty dandruff where all else has failed.
More as a matter of interest than practical use, perhaps: it's also a traditional remedy for TB, which, with the emergence of antibiotic resistant strains of the disease, the medical profession are now investigating.
Interesting hyssop fact No 2 - it's also the herb which gives the liqueur Chartreuse its distinctive colour and flavour