You'll probably know real quick if it isn't going to work out! My friend runs a rescue group and this is what she's told me: the best first meeting is where the dogs meet each other, sniff, yet respect one another's space. Even if you don't see them play together doesn't mean they won't like each other. They will eventually warm up to each other. The best match is of the opposite sex - I'm not sure you mentioned whether it was a male or female? Make sure the first meeting is on neutral territory for both dogs, don't force them to play, and if they're getting too wild with each other, try going for a walk with them.
If you do get the dog, one way to ease the tension (especially if you are going to ride back 4 hours with both dogs in the car?) is to take them for a nice long walk together first so that they get used to being around each other, yet not in each other's faces. Also, for the first little while when you've got the new dog, it'll help Oscar if you can spend some one-on-one time with him away from the new dog.
Maybe this can reassure you: Last october I adopted a male boxer as a companion for my female. It was love at first sight - then again, he was young and super playful, so they were always together romping around, it was wonderful. Sadly, he passed away in December. After that, i fostered another male boxer. He was extremely rude with other dogs. He came here and it was pretty tense at first, until he learned some manners when playing with Kash, and then they liked each other a lot (she let him finish her cookies - that says a lot!). Eventually he was adopted, and a week later, I brought home another male boxer. He came here and was very very shy/reserved. He didn't play with Kashmir for about 1 week. Then he started gradually to come out of his shell. It's been 3 weeks now, and it's like they're best pals! So... my point is, if they seem ok with each other at first, chances are, it'll work out great in the end! I never thought I'd find another match like my first, but it turns out I was wrong - Kashmir likes them all!!