Bhashya
तानि अक्षराणि अभ्यतपत्, तेभ्य अभितप्तेभ्य ॐकारसप्रास्रवत् ।तत् ब्रह्म कीदृशमिति, आह— तद्यथा शङ्कुनापर्णनालेन सर्वाणि पर्णानि पत्रावयवजातानि संतृण्णानि निविद्धानि व्याप्तानीत्यर्थ ।एवम् ओंकारेण ब्रह्मणा परमात्मनप्रतीकभूतेन सर्वा वाक् शब्दजात संतृष्णा—‘अकारो वै स-र्वा वाक्’ इत्यादिश्रुते ।परमात्मविकारश्च नामधेयमात्रम्इत्यत ओकार एवेदं सर्वमिति ।द्विरभ्यास आदरार्थ ।लोकादिनिष्पादनकथनम् ओकारस्तुत्यर्थमिति ॥
In plain words
Those imperishable syllables were heated by tapas, and from them, thus heated, the sacred syllable Om flowed forth. How is that Brahman to be understood? The text explains: Just as every part of a leaf is threaded and pierced through by a stalk or a needle, so too is every sound and every form of speech pierced through and pervaded by Om, which is Brahman and the symbol of the Supreme Self. As the scripture says, 'The letter 'a' is indeed all speech.' Because any transformation of the Supreme is merely a name, this entire universe is nothing but Om. The double repetition is meant to show deep reverence, and the description of how worlds are produced serves to praise the syllable Om.
Close to the Sanskrit
Those imperishable syllables he heated intensely; from those heated ones the syllable Om flowed forth completely. What is that Brahman like? He says: Just as all leaves and leaf-parts are pierced and threaded through by a needle or a leaf-stalk, even so all speech and sound-aggregates are pierced by Om, which is Brahman and a symbol of the Supreme Self—based on the scriptural text 'The letter 'a' is indeed all speech.' And since a modification of the Supreme Self is a mere name, therefore the syllable Om is indeed all this. The twofold repetition is for the sake of reverence. The statement of the generation of worlds and so forth is for the purpose of praising Om.