B.G 8.12 and 13
सर्वद्वाराणि संयम्य मनो हृदि निरुध्य च। मूर्ध्न्याधायाऽत्मनः प्राणमास्थितो योगधारणाम् ॥१२॥
sarvadvārāṇi saṁyamya mano hr̥di nirudhya ca। mūrdhnyādhāyā'tmanaḥ prāṇamāsthito yogadhāraṇām ॥12॥
[सर्वद्वाराणि (sarvadvārāṇi) - all the gates (senses); संयम्य (saṁyamya) - having restrained; मनो (mano) - the mind; हृदि (hṛdi) - in the lord Nārāyaṇa; निरुध्य (nirudhya) - having fixed, restrained; च (ca) - and; मूर्ध्नि (mūrdhni) - in the head; आधाय (ādhāya) - having placed; आत्मनः (ātmanaḥ) - of the self; प्राणम् (prāṇam) - the breath; आस्थितः (āsthitaḥ) - being established; योगधारणाम् (yogadhāraṇām) - in the practice of yoga;]
Having restrained all the gates, fixing the mind in the lord Nārāyaṇa, placing the breath in the head, one remains established in the practice of yoga.
ओमित्येकाक्षरं ब्रह्म व्याहरन् मामनुस्मरन्। यः प्रयाति त्यजन् देहं स याति परमां गतिम् ॥१३॥
omityekākṣaraṁ brahma vyāharan māmanusmaran। yaḥ prayāti tyajan dehaṁ sa yāti paramāṁ gatim ॥13॥
[ओम् (om) - the sacred syllable Om; इति (iti) - thus; एकाक्षरम् (ekākṣaram) - the one syllable; ब्रह्म (brahma) - the Supreme Reality; व्याहरन् (vyāharan) - uttering, pronouncing; माम् (mām) - me; अनुस्मरन् (anusmaran) - remembering, meditating upon; यः (yaḥ) - whoever; प्रयाति (prayāti) - departs, goes forth; त्यजन् (tyajan) - abandoning; देहम् (deham) - the body; सः (saḥ) - he; याति (yāti) - attains, reaches; परमाम् (paramām) - the supreme; गतिम् (gatim) - destination, state;]
Whoever, uttering the one-syllabled Om, which is Brahman, and remembering Me, departs leaving the body, attains the supreme destination.
Gīta Tātparya 8.12 and 13
'ekākṣaram' - the Supreme Brahman.
एकाक्षरवाच्यत्वात् एकाक्षरं परं ब्रह्म ॥१३॥
ekākṣaravācyatvāt ekākṣaraṁ paraṁ brahma ॥13॥
[एकाक्षरवाच्यत्वात् (ekākṣara-vācya-tvāt) - due to being denoted by the single syllable (Oṁ); एकाक्षरम् (ekākṣaram) - the single syllable (Oṁ); परम् (param) - supreme, highest; ब्रह्म (brahma) - the Absolute, the Supreme Reality;]
Since the Supreme Brahman is conveyed by the single syllable (Oṁ), he is called 'ekākṣaram', i.e. single syllable.
