As mentioned in your other topic, there is no way to open .OAM as an usable project within AE. .OAM are packaged exports, not AE projects. You will need the .AN file (and all related projectfiles) to use them in AE. You can see what's inside an .OAM by renaming it to .ZIP and extracting the contents. However, you won't be able to change them in-depth. Content-related changes (HTML, images, etc) is possible though, however in that case you are simply editing HTML (as opposed to using AE).
Actually you can do it.
step one - change oam to zip.
step two - unzip.
step three - follow the directions from my tutorial here: