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.AOI File Extension

File Type	Dynamics AX Application Object Index File
File Format	File
Common?	No
File Description	Contains an index of all object data (.AOD) files referenced by a Microsoft Dynamics AX application; provides faster access and search capabilities for object data used by the program.
Program(s) that open aoi files	Windows: Microsoft Dynamics AX

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/aoi