Try this:  take your OPC and filter it through a few layers of unbleached coffee filters and see if that makes a difference.   I am a firm believer in extra filtration of all oleander extracts to make sure that most or all of the organic compounds that can cause some of the more common usually-mild-yet-annoying  side effects are filtered out.  I do this on my home-made extract as do several others who follow that recommedation and we have all had virtually no side effects beyond perhaps a bit looser stool than normal.

Insofar as taking measures to protect your liver: Absolutely!  Not protecting it from oleander, which is very good for the liver, but from the toxins that may be expected to be released due to bad cells (such as cancer) being killed off.  Did the instructions not advise you to begin with a good liver cleanse?  You really should (and if it contains psyllium husks, do it without taking the OPC at the same time).  And then take milk thistle and perhaps alpha lipoic acid, selenium and/or turmeric supplements (liver protectors and rebuilders, as well as cancer fighters) PLUS coconut oil and organic soy lecithin.  The best place to buy organic soy lecithin is Mountain Rose Herbs - who have a unique patented process that avoids the harsh chemicals used for other soy lecithin versions (even ones labeled as organic and natural).

Liver health and cleansing is essential to everyone, especially those of us who are combating a health problem.

May I ask what you are using the OPC for?  If it is cancer or hep-C, you should see these articles:

Hepatitis-C - The Hepatitis Epidemic and Natural Remedies That Can Help

A Natural Anti-Cancer Protocol

An excellent place to find information about cleansing is:

http://www.mnwelldir.org/docs/detox/clean.htm

Tony