Improve the performance of the following code using Python's in-built functions.
s = 'Google'
count = 0
for i in range(len(s)):
    if s[i] == 'o':
        count+=1
s = 'Google'
# improved code
# use of built-in functions to count the number of occurrences of 'o' in a string
count = s.count('o')

print('Number of occurrences of "o": {}'.format(count))