If someone who does not even know the meaning of the words recites Psalms, he is engaged in Written Torah.
He is dealing only with the letters of the text (even though he may not even be able to read), just as the sofer or scribe, who copies the Torah letter by letter is involved with Written Torah.
But as soon as someone begins to think about what the words mean, he becomes engaged in something else, in Oral Torah.
–Rabbi Adin Steinsaltz
From "Elixir of Life" in In The Beginning by Rabbi Adin Steinsaltz