ONAN or OFAF are the type of cooling arrangements provided as primary ( ON or OF ) and secondary (AN or AF) mechanisms. If you are using forced cooling rather than natural cooling, the power rating of the transformer is enhanced. Normally forced cooling will be provided for power high capacity transformers or for critical load transformers. Retro fitting of forced cooling is also possible if same transformer needs to be retained but higher loads needs to be connected. However initial higher cost, running cost and slightly complex installation considerations needs to be set off against getting an enhanced rating output from the transformer.