Bhagavad Gita 8.11यदक्षरं वेदविदो वदन्ति विशन्ति यद्यतयो वीतरागाः।यदिच्छन्तो ब्रह्मचर्यं चरन्ति तत्ते पदं संग्रहेण प्रवक्ष्ये ॥

The Supreme Being which is symbolised by the sacred monosyllable Om or the Pranava is the highest step or the supreme goal of man.The same ideas are expressed in the Kathopanishad. Yama (the God of Death) said to Nachiketas, The goal which all the Vedas speak of, which all penances proclaim and wishing for which they lead the life of celibacy, that goal (world) I will briefly tell thee. It is Om. Satyakama the son of Sibi questioned Pippalada, O Bhagavan, if some one among men meditates here until death on the syllable Om, what world does he obtain by that Pippalada replied, O Satyakama, the syllable Om is indeed the higher and the lower Brahman. He who meditates on the higher Purusha with this syllable Om of three Matras (units) is led up by the Samaverses to the Brahmaloka or the world of Brahma. (Prasnopanishad)Pranava or Om is considered either as an expression of the Supreme Self or Its symbol like anidol (Pratika). It serves persons of dull and middling intellects as a means for realising the Supreme Self.Chant Om three times at the commencement of your meditation you will find concentration of mind easier.