I have an OTL called cool_thing.otl with several versions inside it:
com.jrcooper::cool_thing::0.1.6
com.jrcooper::cool_thing::0.1.5
com.jrcooper::cool_thing::0.1.4
:
com.jrcooper::cool_thing::0.1.0

According to the docs for Houdini 12+, setting the environment variable HOUDINI_OPNAMESPACE_HIERARCHY to a specific OTL definition will cause Houdini to create nodes of that type by default. For example, I would expect that if I set:
HOUDINI_OPNAMESPACE_HIERARCHY=com.jrcooper::cool_thing::0.1.5

…and then launch Houdini and do this (note the ambiguous reference):
opadd cool_thing

…that I would get a cool_thing node using the 0.1.5 definition. Instead I get the latest version, 0.1.6. I also get the latest version from the usual TAB menu method of creating the node. I have confirmed that the environment variable is indeed set to the given value by interrogating with os.environ.get() in the Python shell.

Does anyone know how to properly use HOUDINI_OPNAMESPACE_HIERARCHY? Am I not understanding the documentation when it says:

You can add a fully-qualified name to the list to make it take precedence even over a later version. For example, adding com.sundae::vines::1.0 to the list would make Houdini use that for an ambiguous vines reference, even if com.sundae::vines::2.0 is available.

Thanks!

I've used HOUDINI_OPNAMESPACE_HIERARCHY a few times couple of years ago and it worked. Now, in H15.5 it seems like it doesn't working. I'll submit it as a bug.

UPDATE:
Answer here: http://forums.odforce.net/topic/27940-houdini_opnamespace_hierarchy/ [forums.odforce.net]

Thank you for posting that reference to the answer.