I have noticed that the numbers of seats in 'I' is the same as 'O' all the time (at least on international flights). It does seems like the it's the same inventory.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Why would you use 2 codes for the same inventory item.</font>
To distinguish those who are flying on an award ticket (so no miles) and those who are upgrading (so they'll get miles)