there is no relation between 16 QAM and OFDM.
OFDM is a technology where you subdivide the available frequency bandwidth like FDM (here it is called subcarrier) with overlapping frequency and transmit the data (16 QAM data in your case) in individual subcarrier. Since the subcarriers are orthogonal, data can be received although the frequency are overlapping. So, data can be transmitted/received in parallel in different subcarriers which increase the data rate.
To know a bit more, google OFDM.