Many of us have combination skin.The oily part sometimes tends to look darker due to a condition called seborrhoeic melanosis.
If you are really bothered about this you can use a cream containing kojic acid or glutathione at night nd use a gel type sunscreen in the daytime
Next Steps
If it is not getting better do consult a dermatologist and doctor will prescribe you stronger creams or can even do procedures to reduce the darkness
Health Tips
please don't use any home remedies containing turmeric or sandalwood since this can further increase the darkness