OllyDbg is able to use dbghelp.dll and symsrv.dll to show extended debug information, such as the module source code (if referenced by the debug information) or module symbols from a PDB file (which can be fetched from the Microsoft Symbol Server for system modules). The problem is that if you turn on this option, module loading becomes much slower. On the other hand, this information is very handy, so there’s a dilemma as of whether to turn it on.
The Symbols on Demand plugin provides the best of both worlds: it disables loading of this extended debug information by default, but allows to load it explicitly for any module, at any time. Using this approach, loading is still fast, but if you need to load extended debug information for a module, you can easily do that.
OllyDbg v1.10 and v2.01 are supported. For, OllyDbg v1.10, there’s additional functionality: you can set the symbols search path, which is set by default to SRV*.\Symbols*http://msdl.microsoft.com/download/symbols. You can also choose to retrieve undecorated symbol names. These options can be set in the INI file of OllyDbg, in the plugin’s section.