Since the Chandogya Upanishad was written a few hundred years before Gautama Buddha, there's no doubt that OM is older than the Buddha. It doesn't matter whether it's a hundred years older or a thousand years older.
As far as I know, OM does not appear anywhere in the Pali suttas or the other Early Buddhist Texts. Also, Gautama Buddha himself had never used or mentioned OM, to the best of my knowledge.
In the Theravada tradition, phrases of homage to the Buddha like "namo tassa bhagavato arahato samma sambuddhassa" do not start with OM.
So, where is OM used in Buddhism?
It appears to be used mainly in Vajrayana or Tibetan Buddhism - the most famous one being "OM Mani Padme Hum". There are others like "OM Tare Tuttare Ture Soha".
So, where does "OM Mani Padme Hum" come from?
The wikipedia page on "Kāraṇḍavyūha Sūtra" states:
The Karandavyuha Sutra is a Mantrayāna sutra that was compiled at the
end of the 4th century or beginning of the 5th century CE. According
to the Kāraṇḍavyūha Sūtra, the sun and moon are said to be born from
Avalokiteśvara's eyes, Shiva from his brow, Brahma from his shoulders,
Narayana from his heart, Sarasvati from his teeth, the winds from his
mouth, the earth from his feet and the sky from his stomach.
The sutra introduces the Buddhist mantra, Om Manipadme Hum, which it
states can lead to liberation (moksha) and eventual Buddhahood. A.
Studholme sees this famous mantra as being a declarative aspiration,
possibly meaning 'I in the jewel-lotus', with the jewel-lotus being a
reference to birth in the lotus made of jewels in the Buddhist
Paradise, Sukhavati, of Buddha Amitabha. The mantra is the very heart
of Avalokitesvara (the supreme Buddha of Compassion) and can usher in
Awakening.
So, "OM Mani Padme Hum" appeared in Buddhism, through the Karandavyuha Sutra, which was compiled almost a thousand years after the passing of Gautama Buddha.
And, it's a mantra for the Bodhisattva Avalokiteśvara, from whom emanates the sun, the moon, Shiva, Brahma, Narayana, Sarasvati, winds, earth and sky.
Now, let's just make it clear that the Early Buddhist Texts and Gautama Buddha himself have never mentioned Avalokiteśvara, OM, Shiva, Narayana or Sarasvati. This is to the best of my knowledge.
What does OM mean in Buddhism?
The 14th Dalai Lama gave a lecture on the meaning of: OM MANI PADME HUM. He said:
It is very good to recite the mantra OM MANI PADME HUM, but while you
are doing it, you should be thinking on its meaning, for the meaning
of the six syllables is great and vast. The first, OM, is composed of
three pure letters, A, U, and M. These symbolize the practitioner's
impure body, speech, and mind; they also symbolize the pure exalted
body, speech and mind of a Buddha.
The OP asked:
OP: Is its use the same as in Hinduism? If you recite the OM, do you (or not) think it is connected to a Super Consciousness as
described in the Vedanta?
So, the answer is that OM in Buddhism has nothing to do with Super Consciousness or Vedanta. Rather, this is what it means in (Vajrayana or Tibetan) Buddhism: "These symbolize the practitioner's impure body, speech, and mind; they also symbolize the pure exalted body, speech and mind of a Buddha."
OP: Is it difficult to describe the philosophical basis, if the practice is somehow derived from concepts and knowledge from a text
whose date is from before Gautama?
Mmm... not really. The Dalai Lama has explained the philosophical basis above.
