This depends on platform of course, since legacy C systems on every platform have different characteristics.
_IF_ the object type is compatible, you can link it by placing {$link mylib.o} in your source.
You must then also satisfy all linking requirements of mylib.o (should be in the documentation provided with the .o or its compiler).
If it is Linux or *BSD, using unit "initc", might be a convenient workaround to link the basic legacy C libraries. Mac OS X even links to it by default.