What is a ODEX File Type?
ODEX files are the optimized versions of .DEX files containing the executable code d' an Android application. While DEX files run generically on all Android devices, ODEX files are optimized for the specific device on which they reside. Not only does it help the loading speed, but it also reduces the space required for the application.
Android creates ODEX files for applications before they are run, and they contain the same filename prefix as the corresponding APK file, for example MyApp.apk and MyApp.odex. The data in ODEX replaces the data used in the equivalent DEX file (classes.dex) stored in the APK file.
Dalvik is the name given to the virtual machine running the compiled code for Android applications. An optimized DEX file, or ODEX file, is therefore optimized for execution by the Dalvik virtual machine.