What should be the cut off frequency of a Filter operating on an OFDM signal?
You don't need that filter. Your previous questions still indicate you shouldn't be using a filter. OFDM is a filterbank in itself, and applying a filter beforehand will increase delay spread, and thus you get ISI¹.

Sure, with a filter, you'll save on the noise power you'd get in carrier sidelobes, but that should typically be lower than what you lose in ISI.

So, no, you don't apply a different linear filter. You simply ignore the FFT bins that don't contain signal.


¹ you'll get ISI under the assumption that your OFDM system is designed with the minimum amount of subcarriers needed to compensate the maximum expectable delay spread on the channel. That makes a lot of sense because using significantly more subcarriers has no advantage, but makes your system depend on a longer coherency time. If you convolve the channel output with your receive filters, you get a "convolved channel-filter", with a length that is thus longer than your channel, and thus worse than your OFDM system can deal with.