As the tutorial says, the individual methods which use OpenOptions define their defaults.
Eg. Files.newBufferedWriter defines the following defaults:
If no options are present then this method works as if the CREATE, TRUNCATE_EXISTING, and WRITE options are present. In other words, it opens the file for writing, creating the file if it doesn't exist, or initially truncating an existing regular-file to a size of 0 if it exists.