When immunity goes down (diet, stress, candida...)
your skin/scalp immunity goes down also and
some pathogens like candida but most likely Malassezia take advantage. if its malasezzia i would be carefull with any kind of oil as "the fungus requires fat to grow" wikipedia. i used to apply olive oil or even castor oil and made it worst.
i had also good results from drops of TEE TREE OIL added in the shampoo or whatever you are using.