There is only one file type that uses the .OAT extension:
Android Optimized Application File
An OAT file is created by the Android operating system in order to speed up the loading time of an Android app (.APK file). When an app is installed, Android automatically optimizes app data and creates a corresponding OAT file. Android uses this file to load the app more quickly, which creates a better experience for the user.
OAT files are typically saved to the following directory on the Android device:
Android performs these optimizations using a tool called dex2oat. When you install an application on a device running Android 5.0 or later, dex2oat converts the Dalvik code located in the .ODEX file to native code and stores it in an OAT file. Therefore, each app will have a corresponding OAT file. dex2oat is typically located in the following directory on the Android device:
NOTE: OAT files were introduced with Android 5.0, which is codenamed “Lollipop" or “L.” Previous versions of Android use .ODEX (Optimized DEX) files instead of OAT files for optimized executables. The new Android 5.0 runtime, called ART (Android Runtime), uses an ahead-of-time (AOT) compilation method, which improves upon its predecessor’s method of just-in-time compilation. ART replaces the previous runtime, which is called Dalvik.
boot.oat - This file is created whenever a device's system is upgraded or when the app is booted up for the first time after being purchased. The boot.oat file is referenced by an app when the app calls a framework application programming interface (API) method.
Updated: January 27, 2016