The orbital angular momentum (OAM) is considered as a new degree of freedom (DOF) that could be utilized to enhance the capacity of wireless communication systems. However, it is known that uniform circular array (UCA)-based OAM in line-of-sight channels is a special case of the multiple-input multiple-output (MIMO) system. In this letter, we investigate the performance of UCA-based OAM communications in a “spatial keyhole” channel and reveal that in contrast to traditional MIMO, UCA-based OAM may provide additional DOF. In particular, the phenomenon that diffraction at the “spatial keyhole” does not change the helical phase structure of OAM modes provides a promising way to overcome the keyhole effect for traditional multi-antenna wireless communications.