You can use optparse-callbacks to achieve this.
Here is how it wiill work for your use case.
parser.add_option("-o", action="callback", dest="output", callback=my_callback)
def my_callback(option, opt, value, parser):
if len(parser.rargs) > 0:
next_arg = parser.rargs[0]
if not next_arg.startswith("-"):
# Next argument is not another option
del parser.rargs[0]
setattr(parser.values, option.dest, next_arg)
return
# If not processed, set the default value
setattr(parser.values, option.dest, "output.txt")