You have good oral hygiene if you brush your teeth often and keep your breath fresh. You have bad oral hygiene if your breath smells bad and your teeth are not in good order. It doesn't refer to how teeth and gums look unless they are diseased and smelly. In short, oral hygiene refers to the state of a person's mouth. I have not heard this term to refer to the way someone uses language. Not in British English anyway.

hygiene = keeping clean (taking a shower, ...) oral hygiene = keeping your mouth clean (brushing your teeth, ...)

Oral hygiene is the practice of keeping one's mouth clean and free of disease and other problems (e.g. bad breath) by regular brushing of the teeth (dental hygiene) and cleaning between the teeth. It is important that oral hygiene be carried out on a regular basis to enable prevention of dental disease and bad breath. Source: https://en.wikipedia.org/wiki/Oral_hygiene

Good oral hygiene means brushing your teeth, flossing, using mouthwash and getting regular check-ups from your dentist.

Paulo - do you REALLY think its necessary to go into the finer points of subjective meanings of the word hygiene in a question where the person simply asked the meaning of "oral hygiene"? Being that the person asked specifically about the words "Oral" and "Hygiene" juxtaposed, I think we can count on the fact that the answer he gave was correct.in that capacity.