It's value is organic matter, and likely oat and weed seeds. However you can't do anything about the seeds anymore so might as well till it under.

I've used oat straw,wheat straw,alfalfa, wood chips, and the best thing i've used so far is a paper called weed guard. I'm not sponsoring them, thats just my opinion. For me it works great. It lasts about 6-9 months, and it decomposes. You can find more in here on garlic: Jubilee Produce

I like wheat straw, but I run it through the lawn mower first to break it up so that it breaks down faster. It is the nature of wheat straw to carry many fewer weed seeds than oat straw does. Plains wheat normally grows in far purer stands than does Midwestern oats. So there are fewer weeds to seed. When you till it in, remember straw is carbon; so you will need some sort of nitrogen to balance it.

Another mulch I like is the last lawn mowing of the season with the tree leaves left down; so they get chopped into it. You get a good green/brown mix if you let the grass grow up some before cutting it that last time.

I use oats as green manure for my garlic, onions. Till it in. It helps the soil.

Oats is an excellent green manure. Plant as early as possible for best stooling, mow short for excellent chopped mulch but do that well before seed ripening of either the oats or any other involved plant that may be part of the stand, and you may even get some regrowth. It is an excellent annual grass with plenty of roots and top growth to be tilled under. Oats is quite frost hardy, but not completely winter hardy; so some more southern gardens may have it over winter, but it will not be dependable for more northern gardeners, although it is often successfully planted even into frozen ground; so it oats can be planted extremely early in the spring and actually does best then.