OptionParser does not enforce required argument

I'm not seeing the difference between specifying a required argument and
an option argument in OptionParser other than they used '<<' in the
assignment of the former. When I don't give an output_file it runs fine
until it needs to use the filename and then gives a nil error. When I do
specify the filename I get an error for using the '<<' operator. Command
line output for both cases given below the code:

def self.parse(args)
      options = OpenStruct.new
        String usage = "Usage: itos_logs_dms -o <output_file>
[Options]\n\nThe following script is used to filter a set of ITOS log
messages from the log\ndatabase and format them for parsing by
Attention!. The fields will be tab\ndelineated and any tabs in the ITOS
event message will be replaced by 8 spaces.\n"

        opts = OptionParser.new do |opts|
            opts.banner = usage
            opts.separator ""
            opts.separator "Specific options:"

//This is supposed to be the required argument
            opts.on("-o", "--output OUTPUT_FILE", String, "Name of
OUTPUT_FILE is a required argument.") do |o| options.output_file << o
end

            opts.on("-i", "--tag_id [TAG_ID]", String, "Retrieves only
the messages that are tagged with the given tag_id.") do |i|
options.tag_id = i end
            opts.on("-s", "--severity [SEVERITY]", Integer, "Retrieves
only the messages that are >= the given severity (0-5).") do |s|
options.severity = s end
            opts.on("-t", "--timestamp [TIMESTAMP]", Time, "Retrieves
only the messages that are >= the given timestamp (yyyy-mm-dd
HH:mm:ss.SS)") do |t| options.timestamp = t end
            opts.separator "Common options:"
            opts.on_tail("-h", "--help", "Show this message.") do puts
opts exit end
        end

        opts.parse!(args)
        options
    end

# No Option Given:
<stsmitz@ptolemy-redhati386 ~> ./itos_logs_dms.rb
./itos_logs_dms.rb:45:in `+': can't convert nil into String (TypeError)
        from ./itos_logs_dms.rb:45:in `createFile'
        from ./itos_logs_dms.rb:75:in `<main>'

# Option provided (two ways):
<stsmitz@ptolemy-redhati386 ~> ./itos_logs_dms.rb --output log_test1
./itos_logs_dms.rb:17:in `block (2 levels) in parse': undefined method
`<<' for nil:NilClass (NoMethodError)
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1271:in `call'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1271:in `block in
parse_in_order'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1258:in `catch'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1258:in
`parse_in_order'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1252:in `order!'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1343:in `permute!'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1364:in `parse!'
        from ./itos_logs_dms.rb:25:in `parse'
        from ./itos_logs_dms.rb:75:in `<main>'


<stsmitz@ptolemy-redhati386 ~> ./itos_logs_dms.rb -o log_test1
./itos_logs_dms.rb:17:in `block (2 levels) in parse': undefined method
`<<' for nil:NilClass (NoMethodError)
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1302:in `call'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1302:in `block in
parse_in_order'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1258:in `catch'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1258:in
`parse_in_order'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1252:in `order!'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1343:in `permute!'
        from /usr/local/lib/ruby/1.9.1/optparse.rb:1364:in `parse!'
        from ./itos_logs_dms.rb:25:in `parse'
        from ./itos_logs_dms.rb:75:in `<main>'

<stsmitz@ptolemy-redhati386 ~> ruby -v
ruby 1.9.1p376 (2009-12-07 revision 26041) [i686-linux]

Thank you for your help in advance.
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