The OP184 and AD820 share a similar pin out but they vary significantly in
their bandwidth, offset, and noise. I would not consider them to be drop in
replacements. The decision will be made based on the specific applications. You
need to analyse your circuit requirements and determine whether the OP184 and
AD820 can meet these requirements.
The Null function on the AD820 is used to trim the input offset voltage. It is
an optional function (only available on the DIP package) which is described in
figure 19 of the REV.C datasheet.
A 20K resistor between pins 1 and 5 with the wiper to the negative terminal.
The null range is +-1mV which is good for this part.