I found a YouTube video of a public domain opera, and I want to use the sound portion in a musical parody. I will only use parts of it, alter it dramatically, and wish to claim "Fair Use." Is it indeed, Fair Use under these circumstances? Do I have any legal obligation to the creator of the sound piece? He used public domain arrangements, and placed it on YouTube. I would be happy to give him credit for his work though. Thank you!
The opera may be in the public domain, but unless the performance is from several decades ago, which I assume is not the case, the performance is not in the public domain. The video therefore has copyright protection of its own. The use to which you want to put the video does not sound like fair use to me, although as the other answer notes that's impossible to determine without knowing more than you've told us, but the fact that the composition being performed is in the public domain is not a particularly important consideration in the analysis.
The standard answer is, there is a concept of Fair Use under which one might legally copy stuff. But it is impossible to definitively determine whether a particular use would be found to constitute fair use. In lieu of permission, you can (1) take your chances or (2) hire an attorney to give you a detailed analysis based on the specifics of the case and then take your chances. Parody is one of the primary reasons for the Fair Use exception; but a number of Bad Lip Reading musical parodies have disappeared. Option (2) is the safest choice.