I wouldn't be so quick to jump to conclusions that it'll be a huge concern.
At best, someone may ask you out of sheer curiosity in an interview (just like any reasonably curious person would) but I doubt they're going to make a value judgment on you because you work at an oil company.
Even if you were at BP, it would be more out of curiosity -- unless you're a high up exec or someone who is working in the department directly related to the spill, it's not really going to affect how they see you as a candidate for b-school.
You don't need to go out of your way to address it, but if it comes up naturally somehow in a section of an essay or in the conversation in an interview, don't try and avoid it either. Again, it's not something to really worry about when it comes to b-school applications.
It is a pretty damn big oil spill though.
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