Guard Interval in OFDM
There isn't any obvious effect in the frequency domain. However, extending the duration of the cyclic prefix does make your signal more resistant to multipath channels with a large delay spread. As a rule of thumb, you need to size the cyclic prefix duration to cover the entire delay spread. If this condition is met, then the linear convolution that is imposed on your signal by the multipath channel will manifest itself as circular convolution on the cyclic-prefixed signal. This circular convolution can be equalized by simple pointwise multiplication in the frequency domain at the receiver. 

This is why one of OFDM's advantages is often cited as the fact that it permits the use of a "one-tap equalizer." In single-carrier systems, equalization can be one of the harder things to get right, so anything that can simplify that portion of the receiver is a help.