I am doing a black lager and was wanting to do a rest. I read the instructions wrong and placed it in the secondary. Is it too late to do the rest? Is the new added oxygen going to actually make it worse?
You may not need a d rest at all, so in that case you'd be OK. I only do a d rest when I actually taste diacetyl in the beer and that doesn't happen often. The purpose of a d rest is to make the yeast more active so they'll consume dicaetyl. Obviously, that would work better in primary where there's more yeast, but there should still be enough yeast in secondary for it. Between that and the fact that you may not need it at all (do you taste diacetyl?), I think you'll be OK.