Hi Mohamed Ben, OS12 blocks are incapable of sending pilot and training symbols. But OFDM block which was added in Optisystem 13 version is capable of sending pilots and training symbols. So, avoid using OS12 blocks if you need channel estimation in your design.
There are two OS12 blocks. In the block named OFDM Modulator (OS12), you cannot specify the subcarrier locations, perhaps. Whereas, in OFDM Modulator Measured (OS12) block you can specify the location of subcarriers. You can specify which subcarrier is to be null. This can be done in notepad and specifying the path in the OS12 block where the .txt file is located.
Regards.