Verse 1

1. OM! - This Imperishable Word is the whole of this phenomenal universe. Its explanation is as follows,  What has become,  what is becoming,  what will become,  - verily,  all of this is OM. And what is beyond these three states of the world of time,  - that too,  verily,  is OM.

"This Imperishable Word...

The Absolute Brahman,  Self,  or God,  alone is  "Imperishable".  It is well known that the whole of this manifest universe is transient and mortal.  There exists no material form that will not perish with time.  It is Truth as Absolute Knowledge that is imperishable.  The word OM is described as imperishable due to it being the sound or name of the Universe.  Creation,  this Universe,  is that immortal knowledge of the Absolute.

"Is the whole of this phenomenal universe"

The word OM,  described above as the sound of eternal knowledge,  is the sound of this Universe becoming physically manifest.  Therefore it is the name of that Knowledge that forms this entire mortal Universe.

"What has become,  what is becoming,  what will become,  - verily,  all of this is OM"

"What has become"  Refers to the experiencing of this manifest creation as it exists,  it is only the created experience that changes due to its transience and mortality.  The Absolute reality itself,  being immortal is eternally unchanging.  The same truth applies to  "what is becoming", Om is the sound of Creation coming into being,  progressing in accordance with its laws to  "what will become",  which includes its inevitable material end.

"And what is beyond these three states of the world of time,  - that too,  verily,  is OM"

All that is created is transient and mortal and therefore subject to the effects of time.
Therefore,  time becomes manifest only with the creation of the material universe,  that is OM.
What is beyond the world of time is what is immortal and the Absolute alone is immortal.  Om is beyond the world of time because whether it is manifest or unmanifest it is held as the eternal knowledge that is the Absolute.