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॥
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॥
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.
Since the Supreme Brahman is conveyed by the single syllable (Oṁ), he is called 'ekākṣaram', i.e. single syllable.