Is it OK to walk my dog in the rain?

Sure it is. Actually dogs like to splash around in the rain, especially in labs and golden retrievers. Of course, the problem will come shortly after you return home all wet and with shades of mud. Small sized dogs may dont like rain because it makes them wet and dirty. 
Get a cheap raincoat, your umbrella and your pet and enjoy a rainy walk without worrying that it well get sick.