Note: I feel the need to put some sort of disclaimer on this post, simply because it's about religion. I'm not sure what to say, though, except that this is just my own, personal experience, summarized for you.
Religion, specifically Christianity, holds something of an odd place in Oberlin. As is the case in many liberal circles, Christianity tends to conjure up images of repression, Jesus camp, and conservatives. Perhaps it's understandable, then, that at Oberlin there are some negative undercurrents toward Christianity.
Now, I don't want to misrepresent things. People won't shun you if you say you're Christian, or laugh in your face, or do anything particularly terrifying or heartless, because Oberlin students aren't like that.
Nevertheless, the fact that I go to church every week isn't usually the first thing that I reveal to people. When I do eventually let it slide that that's where I spend my Sunday mornings, I've never had any negative reactions. The only real uncomfortable moments I've had regarding religion are some of the little comments that people make dismissing Christianity without realizing it. I'm not necessarily bothered by this. I just see it as indicative of the culture at Oberlin and accept it for what it is. If you are particularly religious, though, it's certainly something to keep in mind. Coming to Oberlin was a very big change from being at my high school where it could be assumed that--unless otherwise stated--you identified as Catholic, even if you didn't regularly attend mass.
But do not despair. There are several very active Christian student groups on campus:
Oberlin Christian Fellowship - This group is affiliated with InterVarsity. They meet for large group worship sessions on Fridays and also have small group sessions that meet throughout the week. I've been a few times to the large group sessions, and everyone has been very nice and welcoming. I have several friends who are part of this group and very happy with it.
Ecumenical Christians of Oberlin - A more liberal group than OCF. I've also gone to a few of their meetings and have friends who are a part of this group as well. ECO focuses on discussions (and dinner) and is led in part by the pastors of Peace Community Church, who are both very nice.
Newman Catholic Campus Ministry - I honestly don't know much about this one, so you'll have to look at the website for the Office of Religious and Spiritual Life. Since I'm not Catholic, the only interaction I've had almost no interaction with this organization. I did go to one of their pre-Christmas masses, though, and it was quite nice.
In the end, though, I haven't had much to do with any of the Christian student groups on campus, for the simple reason that they're not the sorts of communities that I'm looking for. They do provide fantastic environments where students can feel safe expressing their beliefs, and they do provide a place to interact with like-minded individuals, but for me, my religion has always been more focused on a church. I like the atmosphere that's provided there--when people of all ages who aren't necessarily part of the Oberlin College community mix.
I've gone to the same church for all four of my years here, and I've slowly become a member of that community, which has been--in a word--fantastic. In good Methodist tradition, I've been invited (and gone) to countless potlucks. It's been more than that, though. I've seen the church through one minister's retirement and the transition to a new minister. I've pledged money to kids participating in the annual CROP Walk, just like when people pledged money for me back in the day. I've made friends with adults who ask me every week how my classes are and what's going on in my life.
My point is, if you're Christian, you're thinking of coming to Oberlin, and you're worried about whether or not you'll fit in, you will. You'll be able to find a community and you'll be fine. Oberlin isn't perfect with regards to Christianity, but my experiences have nevertheless been positive.