1. Theoretically, you're right. When all subcarriers are modulated the (baseband) bandwidth of an OFDM signal is approximately $f_\mathrm{s}/2$. An ideal rectangular filter before A/D conversion would be needed at the receiver to avoid anti-aliasing. As such a filter cannot be implemented in practice, some subcarriers remain unmodulated to make the OFDM spectrum narrower. This zero padding in frequency domain corresponds to oversampling in time domain.
2. An OFDM symbol including guard interval consists of $N_\mathrm{c} + N_\mathrm{G}$ samples each of which has duration $1/f_\mathrm{s}$. The "oversamplingrate" parameter in your formula is not clear to me. As a rate usually has the unit 1/second your formula's result is without unit which is a contradiction to the left hand side of your equation. Any reference for this formula?