Quick answer
Look for 29% or higher. A strong HFFR compound reaches 31% or above.
The oxygen index is the minimum oxygen concentration needed for the insulation to keep burning. Normal air is about 21% oxygen — so any material with an index above 21% cannot sustain a flame in open air once the ignition source is removed. It self-extinguishes.
The higher the number, the harder the wire is to keep alight. It is the single clearest indicator of flame-retardant performance, and any serious manufacturer publishes it.
For reference, APAR Anushakti Fire Protekt publishes an oxygen index of ≥ 31%.