Oversampling in OFDM Why do we need an oversampling in OFDM signal? and how to do it? 
If around half of the Sub-carriers are only used to send data, then do we need to perform oversampling to the signal? Is it necessary to have LPF in OFDM transceiver?
Oversampling in OFDM is usually implemented by modulating some subcarriers at the spectrum margins with zero. This zero-padding in frequency domain corresponds to oversampling in time domain. If only 50% percent of the subcarriers are used, then this corresponds to an oversampling factor of 2. Specifically, if the total number of subcarriers is $N$ and if $M$ of these subcarriers are set to zero, then the oversampling factor is $N/(N-M)$. Note, that the $M$ zero subcarriers have to be arranged in a row, cyclic shifting allowed. Normally, they're arranged symmetrically at the lowest and highest frequencies.

An OFDM transmitter might require a low pass filter in order to keep out-of-band radiation low. The cut-off frequency is usually half of the sampling frequency ($f_\mathrm{s}$).

An OFDM receiver requires a low pass filter before A/D conversion (sampling) with cut-off frequency $f_\mathrm{s}/2$. Otherwise aliasing can occur.