What is meant by ONAN/ONAF?

ONAN refers to "oil natural, air natural circulation"
ONAF refers to "oil natural, air force circulation"

How does ONAN/ONAF affect transformer? Is there any?

It's not an "effect"... it's a rating.

What exactly are you getting at?

actually, we're doing a project called TEXAS INSTRUMENT here in the philippines, and im just new in the field, i wanted to know more about transformer, i took read of the specifications and i read this ONAN/ONAF, and you said its a rating, rating for what? moerover, can transformer classify as ONAN or ONAF?

I did not know the answer, but was curious... so I read a few of the links here (http://www.google.com/search?source=ig&hl=en&rlz=&q=onan+onaf&btnG=Google+Search)to get my answer above. Other than that I am not familiar with the subject.

It's the Type of Transformer cooling.Its considered in choosing the right transformer for the project based on the Load/Capacity & environmental conditions.