OB100 is the "warm restart" OB. See the attachment for more details on restarts factors (from the System and standard functions manual).
In S7-300 CPUs you have only one restart OB, OB100.
Whatever you program in OB100 will be executed once before the start of the cyclic OB1.
So simply set all your SR bits to zero in OB100 and by the time your CPU starts processing the program all your bits will be correctly initialized.