If you don’t have the sesame oil, but you DO have sesame seeds, you can try adding a handful of fresh sesame seeds, toasted sesame seeds, or toasted + ground sesame seeds to really bring in that nutty sesame flavour.  Otherwise, peanut oil is a great substitute for cooking, while tahini / sesame paste will help as a general flavouring for salad dressings.  If you don’t mind so much about matching the flavour profile of sesame oil in cooking, some great substitutes are olive oil or avocado oil.