iCal is the calendaring application preinstalled on every Mac computer as part of Mac OS X. Mac OS X version 10.7 (also known as Lion) provides inbuilt support for connecting to Exchange servers. As a result if you are using Lion you are able to use the Oxford Nexus email, shared calendaring and address book facilities from your Mac via the inbuilt Mail, iCal, and Address Book applications.
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):