The -O switch is used to build a program in a directory other than the default directory. One use for this switch might be to build in a directory on a read/write disk, when the source itself is on a read-only disk. This might be the case if your workstation shares a network-mounted source tree, where you lack permission to write into that tree:

% ./Build -O /u/you/src/obj
Configuration: pfx=, os=SunOS, rel=4.1.4, rbase=4, rroot=4.1, arch=sun4, sfx=
Using M4=/usr/5bin/m4
Creating /u/you/src/obj/obj.SunOS.4.1.4.sun4/sendmail using ../devtools/OS/SunOS

This switch is also useful when experimenting with different settings inside your m4 build file (Section 2.4). Use one directory when experimenting, for example, and the other for production.