Yes, once a day treatment with minoxidil will be effective but without knowing much about you, I can not tell you if what you are discussing is the proper treatment for the hair loss you have. See a doctor to discuss this and then you will know better what to do.
Minoxidil is not well studied at a single dose application. However at a twice daily application, very little minoxidil makes it to the blood stream (<1-2%) indicating that all of its benefits are from local absorption only. Therefore a beneficial effect would be expected with once daily dosing but twice daily dosing is likely to be more effective.
For men, 5% minoxidil use twice daily is more effective than once daily. For women, 5% minoxidil use once daily appears to be as effective as twice daily. Hair oil does not nourish the hair. It can be used if one likes it, but has not specific growth benefits and in some patients may promote acne type lesions or worsen seborrhoea.
Use the medication as directed. If the instructions say use the drug twice a day, that is what has been determined to be the most effective treatment interval. If you are reducing it by half, then logic would dictate that it would not be as effective. Hair oil has no bearing on hair growth.