As long as you use a therapeutic grade oil, (should have an ISO number on the label for traceability), then as per French practice, where essential oils are prescribed by medical doctors, most oils are generally regarded as safe to take nternally -With the exception of absolutes.
I personally take lemon, orange, peppermint, a cinnamon blend, and various others internally.
Oils such as the citrus oils are great for stimulation through the day - aiding focus and clarity of thought, alertness, and as anti-microbials. I find that not only do they penetrate into every cell in the body, enhancing O2 uptake by every cell, but purely in the act of drinking them, the smell / aroma is way more powerful than with purely opical application, and additionally I fin that my sense of touch is also enhanced.
Oils are best taken in a small amount of water - water drives the oils deeper into the cels, faster, heighening their effect. Having said that I tend to put a small amount f water into a glass, add a drop of oil, then top the glass up.
Alternatively with peppermint - or oils high inmenthol, you can add a drop or 2 of oil into a litre bottle, and freeze it. When you then thaw it to drink, the oil will have frozen / crystallised into the water, again heightening its effect, taste and aroma.