After you save your barbed wire as a brush, you can use it to draw with or you can apply it to a path, like your oval.
its easy
if you haven't tried before, take a few minutes to play around, it's not complicated.
Just remember, this is a case where 'size does matter', so save your graphic accordingly.
If you find the brush stretching too much on your path, either make a longer segment for your brush or break your path apart into sections
Both things will reduce the 'stretched' look.