where you located? Oats is generally a spring crop. I would think you'd be better off with winter rye - get production this fall/winter and then again next spring. Why not lime it now? Takes 6 months for it to be effective in soil (less if tilled in). Better off getting pH good before adding too much fert. Pretty sure an hour or 2 for roundup won't do anything - if you till, you'll like kill most weeds since they are likely summer annuals.