Read the packages( 7) manual page for more information about binary packages on OpenBSD.
There are pre-compiled packages available for your release of OpenBSD. These integrate automatically with the version of Apache installed with the OS. However, since there are a large number of options (called flavors) available for this port, you may find it easier to compile it from source using the ports tree. Read the packages(7) manual page for more information in what packages are available.