Did you try the manpages?
Depending on what pager is used on your system (on mine, it's "less"), you can often search the manpage that you're viewing by typing a / followed by the string you wish to search for.
The / key for searching is VERY common... it's used in less, vim, lynx, and many other command line tools. After performing at least one search, just typing / followed by enter finds the next occurence.
Typing /-O0 and pressing enter and repeating the search as necessary would eventually lead you to the following:
-O0 Do not optimize. This is the default.
I can not find related descriptions from gcc manual.
Spend a few more minutes searching. Google is your friend, and the description of this option and all others is easily locatable in the gcc manpage and in the online gcc manual at the gcc site. Look in the section about command line options, and in the subsection about optimization options.