I'm trying to get U omnidrive working on our robot. I've tried just turning the wheels pointing the opposite direction of the others, but that tends to make the robot swing around when strafing. I'm totally new to RobotC, so I have no idea what I would do to correct this. How would I code strafing?

I'm assuming that your wheels positioned like at 45 degree angles like this:

/ \
\ /

I'll assume that you have reflected your motors correctly and all motors moving 127 makes your robot move straight forward.

To strafe to the right, you need to make these two wheels:

/ \
\ /


move forward, and these two wheels:

/ \
\ /

move backward.

Look up how holonomic drives work to get an intuitive sense of this.
If you can't find a good explanation online, I'd be happy to explain.

Sorry, I should have been more specific. The wheels are positioned like this:

| |
- -

That's what I meant by "U" omnidrive.

You can't make a design like that strafe straight. There is a driving force on only one side of the robot, so it will always turn. Imagine pushing on only one corner of a tissue box, and trying to make it move straight.