bhagavad-gita asks to chant OM

bhagavad-gita 8.13 asks to chant OM

ॐ इत्येकाक्षरं ब्रह्म व्याहरन्मामनुस्मरन् |

यः प्रयाति त्यजन्देहं स याति परमां गतिम् || १३ ||

 OM ityekaksharam --- 

this verse says chant Om to reach supreme god or to attain moksha.

bhagavat Geeta 7-8

and in another Verse from bhagavat Geeta 7-8 he says he is that "OM" in all Vedha Mantras/Sounds. 

"Praṇavaḥa Sarvavedeṣhu Shabdaḥa."
here Pranavaha is called OM Mantra. 

data from:

https://prabhupadabooks.com/bg/8/16

https://www.holy-bhagavad-gita.org/chapter/8/verse/13

https://srimadbhagavadgitahindi.blogspot.com/2015/10/8-8-13-bg-8-13-bhagavad-gita-as-it-is.html?m=1