My case :
Year: 2005 Summer..
Attending Community College, left HIST- II and GOV- II to get Ass. Deg in General Science.
Took both classes on summer, applied OPT, Failed in HIST - II (took class only to get OPT)
Still got OPT, worked for year ( during that time trans to 4 - year)..
Later took HIST-II there and transferred back to community, got my 4 year and ass degree at the same year.

My understing -- You can still get OPT and work with 6 or less credit hours left for degree. SO I think U failing one subject will not stop u from getting OPT. However, U might not get the degree certification from college until u take that course again. This is important because you are not fully graduated, and some companies do graduation check as well..

Going back to college in OPT is grey-area. At least at my time (2007, 2008), there was no definite answer. My advisor suggested me to take only half credit hours...Technically OPT is a F-1 status.
Year 2008 Fall" took 6 credit hours in community college while in OPT...

Hope this helps...by the way your advisor should answer all your concerns....if not go to another advisor .......