iCal is the calendaring application preinstalled on every Mac computer as part of Mac OS X. The version of Mac OS X that was released in September 2009, Mac OS version 10.6 also known as Snow Leopard, provides inbuilt support for connecting to Exchange servers. As a result Snow Leopard users are able to use the Oxford Nexus email, shared calendaring and address book facilities from your Mac via the inbuilt applications. So, Mac users who are running Snow Leopard can use Nexus calendaring via iCal on their Mac.
If you have already configured Mail or Address Book to use Nexus the chances are that your iCal application has already been set up for you to use with Nexus. That is because when you add an Exchange account to Mail or Address Book the default behaviour is to also set up the other components that make up the suite of programs that can be used with Exchange at the same time. This can be seen in the Account Summary screen that you get, for example, when configuring Mail (note the "Also set up:" check boxes):