Oral thrush occurs when a yeast infection develops on the inside of your mouth and on your tongue. This condition is also known as oropharyngeal candidiasis. Oral thrush is caused by the fungus Candida albicans.ral thrush occurs when a yeast infection develops on the inside of your mouth and on your tongue. This condition is also known as oropharyngeal candidiasis. Oral thrush is caused by the fungus Candida albicans.