The offset for circuits such as op-amp depends on the mismatch between various circuit components. This offset can be estimated based on the mismatch data provided by the fab. Normally, the fab would provide the mismatch data for transistor pairs laid down in specific layout schemes. Seeing the data, the designer can decide the best possible layout configuration and transistor size for best offset performance. You will have to check the fab's mismatch data for all components such as transistors, resistors, capacitors that are used in your circuit.
Sometimes, error in biasing and incorrect sizing may lead to offset [Ex: Basic 2 stage op-amp]. The only solution to avoid such offset is careful design.
I hope I could be of some help.