Can Oblivion Mod Manager be used to install mods that aren't packaged as OMODs? If so, how?

Yes, you create an OMOD out of the archive. Have a look at the help function in OBMM - it tells you how to create OMOD's.

I also have a tutorial http://darkcreations.arthmoor.com/forums/index.php/topic/39-using-obmm-to-install-mods/ that goes into excruciating detail on the subject

You can convert them to OMODs... Just read the documentation provided with the tool.
Many people here say that BAIN from Wrye Bash is much better than OBMM, though. I personally don't use it, but it has some functions OBMM lacks (as restoring overwritten files when uninstalling a mod). It can install archives and folders directly without special compressed archives such as OMODs and can automatically manage your install order.

Ohh, ok. Is Bain more user-friendly?

Only one way to find out

I would say it has a leaning curve, similar to what OBMM has. Would recommend just jumping to BAIN rather than using OBMM, then finding BAIN is better and learning it.

Refer to http://www.tesnexus.com/downloads/file.php?id=35230.

Ok, thank you every. I guess my previous advice was a little outdated.

First, I have asked waaaay more stupid questions than that on this forum (and have one pending on the UOP thread now), but people have always responded with useful suggestions.

Second, there are several mods which are already packaged as OMODs - some of them exclusively so. Therefore, I would recommend continuing to use OBMM for some of those, but understand that some may take exception with that advice.

Third, that said, since Wrye Bash has so many other incredibly useful tools, I would strongly recommend diving in and learning BAIN for all of the non-OMODs wandering that you will want to use.

Cheers

There is (or there should be) an utility which unpacks OMODs... For BAIN users.
But I don't remember how is it called, or if it is incorporated in Wrye Bash.

Screen from the WBPG http://www.tesnexus.com/downloads/images/35230-1-1317080659.png