Legume nitrogen credit and forages

I raise spring and summer forages.  I have often debated adding legumes into my rotations for soil improvement and nitrogen credit to following crops.  I get many mixed answers when it comes down to how this works.  For example, in a mix of oats and peas, will the peas produce any nitrogen that the oats could use?  Or will the nitrogen only be available to the following crop after the peas are killed?  I have ways to utilize legumes such as peas and vetch, just not sure what to expect.  Hopefully someone can explain the system better.

Legume nitrogen credit and forages