Need for Oversampling in OFDM and symbol time determination
As the questioner refers to my code at github in the comments, I'd like to clarify. 


There is no need to apply oversampling in OFDM, nor is oversampling required to recover input signals. However, in practical implementations you have to consider some additional constraints regarding the signal's bandwidth. The WARP v2 radio board is clocked with 40 MHz, i.e., the Nyquist frequency is 20 MHz. However, the WARP v2 architecture limits the generated baseband signal's bandwidth to ~10 MHz by TX and RX lowpass filters. This is a meaningful measure to avoid unpleasant aliasing effects. Hence, the OFDM signal's bandwidth should be limited to 10 MHz in the baseband. You can easily achieve this by padding all subcarriers above 10 MHz with zeros. This will work perfectly well. 
An alternative (but probably rather uncommon) way to reduce the bandwidth is to apply 2x oversampling while using the majority of all subcarriers. I implemented oversampling in my generic (not-OFDM-related) transmission procedure and finally enabled it in my OFDM code as I could achieve a slightly higher SNR in our office environment with infrequent interference and some narrowband interference. However, you can safely disable oversampling in my code and apply zero-padding instead. 
An OFDM symbol is the time-domain representation of the symbols concurrently transmitted over all subcarriers. It consists of just as many samples as there are OFDM subcarriers since it is calculated as the IDFT of all subcarriers' symbols. When adding the cyclic prefix, you add $N_G$ samples. The duration in seconds is the number of samples times the time per sample. When oversampling, the symbol duration extends according to the oversampling factor. 


Kind regards,
Robin