Indeed, you defenetaly don't need O2 to make the new sights,
the sights are stored in some p3d files, like optika_M16.p3d and so on, all you need to do, is to get one of the this p3d files,or from an existing addon or from the data3d.pbo file, try choosing one near to the one you want cause it will be easier to serve as reference. Also if your getting the p3d file from the game use the DePbo Decryptor on the pbo file.
Than when you have the optika p3d file, use the swap utility in the p3d, and take the texture files mentioned in the utility out of the game, they should be in the data.pbo folder.
Use TextView in those texture files and change the .paa files into .tga ,opening the files with photoshop or paintshop you will than notice that the sight was made using Alpha Channels, using the same technic make your own sight, than do the oposed save the .tga with TextView into .paa, and than use the Swap Utility to replace the pbo nane path into your own addon name, and open a class in the cpp file like:
modeloptics= "mypbofilename\myp3dfilename.p3d", if everything goes as planned you should have the new optics in your weapon