Katha-1-2-15-sarve vedā

Sri Shankara’s Commentary (Bhashya) translated by S. Sitarama Sastri

To him, who had thus questioned, Death explained the thing asked for and also something else, i.e., the worship of ‘Om.’ What praiseworthy goal all Vêdâs without break, i.e., with one voice, declare, to which goal all acts of îapas are intended to lead, and desirous of which men live in the residence of their preceptor, or practise other kinds of Brahmacharya to attain the Brahman, that goal which you wish to learn, I shall tell you briefly. It is this — ‘Om’. The goal which you wish to learn is the goal which is denoted by the word ‘Om,’ and of which the word ‘Om’ is a substitute (Pratîka).

Links
Various resources on the Kathopanishad
Upanishad Series – Katopanishad Commentary by Swami Chinmayananda