It is advisable that the praNavopAsana is learnt directly from the Masters. PraSnopanishad Section V deals with meditation on "OM" .
When we chant "OM" as part of the Vedic Mantras whatever MAtra and Svara is prescribed in that context has to be followed.
Before reciting the Vedas one has to start with : Harih Om
( 4 MAtras)
On concluding the Vedic recitation one has to end with : Harih Om ( 3 MAtras)
GAyatrI Mantra Japa has to be recited mentally as per Vedic svara as ordained in one's SAkhA.
For Mantra japa involving praNava there is no svara but it goes by certain conventions.
For OmkAra Meditations , there are many ways of reciting "OM" which have to be learnt directly from the Yogic Master.