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)