What is the best way to treat oral thrush?

Depends on severity. Many people think they have oral thrush because they see white spots in their mouth but they actually do not have a thrush. Oral thrush is uncommon. It is usually caused by inhaled steroids used in asthma or copd, or by a severe immune deficiency such as hiv. It needs to be treated with an oral antifungal.
Antifungals. Antifungal topical treatment such as nystatin(mycostatin) liquid suspension swish and swallow or Clotrimazole troches dissolve in mouth are the initial treatment. If this fails oral systemic treatment with fluconazole(diflucan) 100-200mg./day will usually be curative. Disease in aids patients can be more refractory.