Starbucks did a similar thing some time ago and came out in support of gay marriage, it worked out very well for them. I seem to remember that their brand popularity actually increased dramatically as a result of it - 30,000 people signed a petition in support of the boycott, but 650,000 people signed a counter petition in support of Starbucks.
This will probably be good for Oreos. They will temporarily lose a few bigoted customers but replace them tenfold with people who support them like Starbucks did, while offering their support to a very important social issue.
Your probably right... but it's worth noting that speciality coffee stores have a more liberal "Core demographic" then Oreos do.
Coffee shopts in general skewing left, and Starbucks always being known for being on the forefront supporting liberal causes, paying there workers well etc.
Oreo sales have declined which would suggest a more conservative base... and there are a bunch of other factors that would seem to lean that way as well... don't feel like fully researching it right now.
I think it may give them a bump, but not of the same level as Starbucks. Only because I think most ore consumers will say "I don't agree, but who cares." and maybe they pick up some new buyers from the left to give them a try... if it becomes big enough they might be bought for specficic liberal events in the next month or so.
Though it might all end up beign completely ignored by the left and lead to a very slight negative dip if it turns out that shooting in Texas was motivated by homophobia.