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An "oop", or "ordinary object pointer" in HotSpot parlance is a managed pointer to an object.  It is normally the same size as a native machine pointer.    A managed pointer is carefully tracked by the Java application and GC subsystem, so that storage for unused objects can be reclaimed.  This process can also involve relocating (copying) objects which are in use, so that storage can be compacted.

The term "oop" is traditional to certain VMs that derive from Smalltalk and Self, including the following:

• Self (an prototype-based relative of Smalltalk) https://github.com/russellallen/self/blob/master/vm/src/any/objects/oop.hh^{Image Added}
• Strongtalk (a Smalltalk implementation) http://code.google.com/p/strongtalk/wiki/VMTypesForSmalltalkObjects^{Image Added}
• Hotspot  http://hg.openjdk.java.net/hsx/hotspot-main/hotspot/file/0/src/share/vm/oops/oop.hpp^{Image Added}
• V8 http://code.google.com/p/v8/source/browse/trunk/src/objects.h^{Image Added} (mentions "smi" but not "oop")

(In some of these systems, the term "smi" refers to a special non-oop word, a pseudo-pointer, which encodes a small, 30-bit integer.  This term can also be found in the V8 implementation of Smalltalk.)

Why should they be compressed?

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