Location: Winnebago County Park 
501 E Sunnyview Rd
Community Park dr,
Oshkosh, WI 54901
OR
Search "Oshkosh Rugby Fields" in Maps 

Instructions for those of you still using a flip phone

From Hwy 41, Take the Jackson St (Hwy 76) Exit. Head east off the ramp (into oshkosh) for approx 1.5 miles. Turn left on Sunnyview Rd (CR-Y) and follow for about .5 miles until you see a sign for "Winnebago Count Park" and turn left. Follow the one-way road to the right until you reach the pitch (it is between 2 softball diamonds).

Closeup of the park.