O.F. (or A.F.)
Abbreviation for ‘other fibres’ (or ‘altre fibre’). Sometimes seen on the composition label of fabrics made of recycled material. Fabrics produced around the area of Prato in Italy are made using yarns spun from a blend of reclaimed wool, or of course other fibres.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary