How to charge magic oil?

No. A person does not have to be Christian to practice hoodoo.
It is just encouraged to keep the tradition the way it is, meaning, don't involve other deities and spirits into the work that are not traditionally used, such as pagan gods, spirits from other ATR's, and the like. The Bible can be excluded in rootwork. Yes, you can use your 1st language.

Generally whatever higher power you believe in is prayed to for protection, or the ancestors are asked for protection. There are a few items that are for protection, such as saltpeter, salt and black pepper, fiery protection recipes, and the like.

It's hard to do hoodoo completely pagan. Hoodoo is Christian African American folk magic. One can only strip away so much of the influence before it is no longer hoodoo. It is perfectly fine to be any religion you want and practice hoodoo, but when you are doing hoodoo works, you don't include these other deities. I heard a Wiccan once explain that he comes back home to the practices of his ancestors when he does conjure and hoodoo works, including their God and their Bible.

Expose it to moon or sunlight, with a prayer.
I use a Purple Energy Plate, and crystals to charge things as well.
You can also ground yourself, imagining yourself rooted deep to the Earth,hold the oil in its jar or bottle and concentrate on energy flowing through it, from one hand, through it, to the other, filtering through you, the Earth, returning charged. This can be done much like one would envision opening chakras.

Thank you so much VooConjure for a great recommended. I thought I might have to stop learning Hoodoo because I'm not Christian at first. I love magic & I believe in magic but one thing I don't do at all is to learn magic to harm other people coz I believe in karma.
Thank you again. I really appreciated your help.

For me praying over the ingredients as I add them then on the finish product I focus my intent and send my energy into it to charge it for the purpose. Catherine Yronwode is a good way to go with her magical oils book, also Ray Marboro is good but the final choice is yours.

The most effective way is to ask a certain spirit to charge it.
Ex.U made a oil for clearing the obstacles ask legba to charge it.