Redheads rarely have oily hair, blondes tend to have it the most often. The sebaceous glands produce too much oil, and there are nutritional aspects which need to be corrected. There are as many as 140,000 oil glands on the scalp. Oil wicks into fine, straight hair the easiest. Wiry hair tends to be the least oily.
Oil production tends to be increased by intense heat and humidity.
Hormonal changes and stress can also affect it. Give yourself a double shampoo, with a rinse in between. This will help you know how well you are doing. Squeeze the juice of two lemons into a pint of water and use it as a finishing rinse. This helps remove soap residues which tend to weigh down the hair.