.OEF File

File extension oef seems to be mainly related to IBM Optim Development Studio, an integrated database development environment for Oracle, DB2, and Informix.

Some sort of data export file produced by the program.

How to open:

It is likely that you work with these .oef files only using Optim.

How to convert:

Considering these files are already exports, you most likely cannot convert them to anything else.