To shorten the time-to-market and reduce the expensive cost of photomasks in advance process technologies, metal-only ECO has become a practical and attractive solution to handle incremental design changes. Due to limited spare cells in metal-only ECO, the new added netlist may often violate the input-slew and output-loading constraints and, in turn, delay or even fail the timing closure. This paper proposes a framework, named MO ESS, to solve the input-slew and output-loading violations by connecting spare cells onto the violated nets as buffers. MOESS provides two buffer insertion schemes performed sequentially to minimize the number of inserted buffers and then to solve timing violations if there is any. This framework has been silicon-validated through industrial designs with more than 1-million instances. The experimental results demonstrate that MOESS can solve more violations with less inserted buffers and less CPU runtime compared to an EDA vendor's solution. The whole framework is built based on a commercial APR tool and can be ported to any other APR tool offering open access to its design database.