Adjustment of the 3456A is via trimmers, unlike the digital calibration of almost every modern DMM. Innovation back then was that the most common trimmers are behind a panel on the front panel, so you don't have to disassemble the meter. But still have to turn trimmers instead of having a computer doing everything. So expect to pay more for adjusting a 3456A than for example a 34401A.
The 3456A has excellent stability. It would not surprise me if it was still well within 1 year specs. Of course that does not give you traceability, if you care about that.
Four-wire measurement cancels the lead resistance, because it separates current-carrying wires (which will drop voltage over the leads due to the I*R drop) from the voltage-sesnsing leads. (almost) no current through the sense leads means almost no errors due to lead resistance. Look up Kelvin sensing.
OC (offset compensated) Ohms is to correct offset voltages (as opposed to resistances) in the circuit under test. The most common source of this is the Seebeck/thermoelectric effect causes small voltages at temperature gradients at dissimilar metal junctions. So if you'd have brass banana plugs connected to copper binding posts, this could produce some microvolts of offset voltage, which can disturb the resistance measurement. Offset compensated Ohms first measures the voltage with the current source turned on, then the voltage with the current source off to measure offset voltages, and subtracts the two values.