A proper noun is a particular name of a person, place or thing. They are written with a capital letter. Articles can be part the proper noun:
I visited the United States last year. (the is part of the proper noun)
I visited Canada last year ("the" is not part of the name of Canada)
It is rare to use "a" with proper nouns, but possible, in hypothetical situations:
Let us imagine a Canada which never became independent from Great Britain and was part of the UK...
But in your example "oak tree" is a common noun. It is not a proper noun. It is very common for proper nouns to have articles.
And similarly "A cow" is a common noun, here referring to one cow (but not one particular cow, any single cow)