What does "wget -O" mean?

Here's the man page of wget -O:

http://www.gnu.org/software/wget/manual/html_node/Download-Options.html#Download-Options

Here's a few examples:

1. wget with no flag

   wget www.stackoverflow.com
   

   Output:

   A file named as index.html

2. wget with -O flag

   wget -O filename.html www.stackoverflow.com
   

   Output:

   A file named as filename.html

3. wget with -O- flag

   wget -O- www.stackoverflow.com
   

   Output:

   Output to stdout

for the manual of wget: use man wget if you are on Unix platform. Else, try "man page wget" on google.

The -O- stand for "Get as a file and print the result on STDOUT"

Depending on your system you should be able to find reference by typing man wget. The GNU Wget man page says this of the -O|--output-document flag:

If - is used as file, documents will be printed to standard output, disabling link conversion. (Use ./- to print to a file literally named -.)

And continues…

Use of -O is not intended to mean simply "use the name file instead of the one in the URL;" rather, it is analogous to shell redirection: wget -O file http://foo is intended to work like wget -O - http://foo > file; file will be truncated immediately, and all downloaded content will be written there.

It's not uncommon to see combined with -q and written as -q0- or -q0 - followed by a uri. It validates against the POSIX standard so, yeah, I'd say it's a standard thing for shell scripting.