Explanation

Here are odd numbers

101, 103, 105, 107, 109, 111, 113, 115, 117, 119, 121, 123, 125, 127, 129

one digit is missing in the first two series 101 and 105

then 

two digits are missing in the next two series 105 and 111

then

three digits are missing in the next two series 111 and 119

then 

obviously, next digit in the series comes after 4 digits ( 119, 121, 123, 125, 127, 129)