The O2 file format is made of a fixed-length part, that provides informations about the structure itself of the file, followed by a variable-length part, that is the content of the file.

Offset	Size	Description
0	16	Globally unique identifier (GUID) of the file format: ABBB4FE2-9C21-450E-80B0-469DFD8A8BFC
16	1	Minor version of the file format
17	1	Major version of the file format
18	1	Encryption flag
19	1	Cipher algorithm identifier
20	1	Hash algorithm identifier
21	16	Initialization vector (IV) of the cipher algorithm
37	4	32-bit CRC calculated on the content of the file
41		Content of the file, compressed with the zlib deflate algorithm, then optionally encrypted

A change of the major version means changes made to the file structure, a change of the minor version means changes made only to the schema of the content of the file.

Version	Changes
3.0	Added IV
2.3	Added <TextType> element
2.2	Unicode encoding support
2.1	Added <Relations> element
2.0	Content compression
1.0	Initial version