Om

Even though naciketa requested yamaraja to focus on the teaching of ātmā, but yamaraja prescribed a special upāsana for him.

sarve vedā yatpadamāmananti tapāmsi sarvāṇi ca yadvadanti |

yadicchanto brahmacaryam caranti tatte padam sańgraheṇa bravīmyomityetat ||1.2.15|| I shall briefly introduce to you that goal which goal all the Vedas propound, for which (they) prescribe all the auterities, and desiring which (the seekers) practice

brahmācārya. It is Om.

Before talking about the upāsana, yamaraja introduces ālambana - the support of omkāra-upāsana, which is Om. tatte padam sańgraheṇa bravīmyomityetat - I shall briefly introduce to you that goal which is desired by you, that is Om. The syllable Om refers to brahman, the reality of the entire universe, which naciketa desires to know.

This Om sarve vedā yatpadamāmananti - which all Veda are having the same goal to reveal uniformly without contradiction. Even though there are many Upanisad, but they have the same purport to reveal that one reality of the entire jagad, which is limitless brahman, and that brahman is me.

Brahman is not just the goal for jñāna-kanda, but it is also the goal of karma-kanda, thus Om tapāmsi sarvāṇi ca yadvadanti - is the purpose of all austerities prescribed by Veda. Even though rituals in karma-kanda are prescribed for the purpose of gaining dharma, artha, and kāma as immediate goals, but we are expected to see their limitations in a course of time, because most of us only gain vairagya after experiencing them. Thus karma-kanda eventually leads us to vairagya towards sense objects, then jñāna-kanda comes into the picture. And Om is the goal yadicchanto brahmacaryam caranti - which is desired by people who take up the discipline of brahmacarya for the purpose of owning up the limitless nature of the self.