How to parse rake arguments with OptionParser

You can use the method OptionParser#order! which returns ARGV without the wrong arguments:

options = {}

o = OptionParser.new

o.banner = "Usage: rake user:create [options]"
o.on("-u NAME", "--user NAME") { |username|
  options[:user] = username
}
args = o.order!(ARGV) {}
o.parse!(args)
puts "user: #{options[:user]}"

You can pass args like that: $ rake foo:bar -- '--user=john'

I know this does not strictly answer your question, but did you consider using task arguments?

That would free you having to fiddle with OptionParser and ARGV:

namespace :user do |args|
  desc 'Creates user account with given credentials: rake user:create'
  task :create, [:username] => :environment do |t, args|
    # when called with rake user:create[foo],
    # args is now {username: 'foo'} and you can access it with args[:username]
  end
end

For more info, see this answer here on SO.