Here is the text straight from After Effects help files:
After Effects 6.5 lets you output to the OMF (Open Media Framework Interchange) format to generate Avid media essence files. Each OMF codec has a fixed frame size and frame rate, depending on the output resolution you choose. The OMF file format doesn’t support audio, so any audio tracks aren’t converted, but you can output audio tracks separately, if desired.
Note: You can also import OMF files. (See “Importing OMF files (6.5)” on page 84.)
To output a composition to OMF:
1. In the Render Queue window, click the Output Module template name to launch the Output Module Settings dialog box.
2. From the Format drop-down box, select OMF.
3. Click Format Options. Choose a format (NTSC or PAL), AVR (Avid Video Resolution), and OMF version.
Note: OMFI version 1.0 is supported for backward compatibility with older Avid systems.