Oversampling in OFDM
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.