Can we work after Oct 1st only with reciept number ?

Currenlty I am in L1 VISA which will expire on 18th Oct with company A, I have applied for H1 B through company B and it is in RFE review stage , Can I start working with company B in this situation or do I have to wait till the VISA gets approved .

You have to wait till your H1 is approved. Until then you are on your L1 visa.