Complex sentence: Frederick Law Olmsted's Olmsted, Vaux and Company was hired in 1869 and in their plan they envisioned Washington Park as the centerpiece of a string of parks and boulevards similar to those they had designed in Buffalo, Boston, and Brooklyn.
Split into simpler sentences: Frederick Law Olmsted's Olmsted, Vaux and Company was hired in 1869 to plan such a park. In their plan they envisioned Washington Park as the centerpiece of a string of parks and boulevards similar to those they had designed in Buffalo, Boston, and Brooklyn.