Hello,it looks like post inflammatory hyperpigmentation ,sun burn or if you've applied any strong cream on that area,uneven skin tone(Dat we can diagnose only after proper consultation)as you have oily skin,Here are few steps to start with for healthy skin:
1.Use mild cleanser twice daily(gel based cleanser)
2.non-comedogenic moisturiser
3.Gel based sunscreen with min. SPF 30 (10am-4pm apply twice).
4.use skin lightening agent containing kozic acid,vit.c,a etc.
Next Steps
visit cosmetologist or dermatologist for proper treatment as it will take time to get desired result and combination of treatments,with few prescribed medicines.
Health Tips
1.Drink min.6-7 glasses of water daily.
2.Eat fruits which are rich in
vitamin c and other antioxidants which will help e.g.oranges,papaya,kiwi,pineapple, broccoli,cauliflower,guava etc.
3.start using night repair cream (medicated).