OBJ file

OBJ files contain the sprites graphics (equivalent to the C ROMs on cartridge systems). Data is compressed however using a simple RLE compression removing runs with a value of 0x00000000. Once decompressed the format is described here: sprite graphics format.

RLE compression

 * First long word value is the compressed data length (/4-1).
 * The system works by checking single long words at a time.
 * When a zero value is found it is stored and the next value is checked. A fill of zero is created to match the value read unless it is zero.

Example
compressed:

0x00000007, 0x11111111, 0x00000000, 0x00000003, 0x22222222, 0x00000000, 0x00000000, 0x33333333, 0x44444444

decompressed:

0x11111111, 0x00000000, 0x00000000, 0x00000000, 0x00000000, 0x22222222, 0x00000000, 0x33333333, 0x44444444

This system will not compress data very well unless there are large areas of zero data within the decompressed sprite data. This is due to a single zero value being inflated by 100% to let the decompressor know it is not a run. To my current knowledge only Art of Fighting uses the OBJ format.

For reference below is a 68000 code snippet from bios TOP-SP1 that handles decompresses.