To calibrate a DO meter you have to follow the directions supplied by the instrument manufacturer.  Usually you calibrate via Winkler titration, water saturated air or air saturated water.  Water saturated air is the easiest to do.  Fill a bottle half way with water, stopper it, and shake it up.  Place your probe in the bottle so it is not touching the water and take a reading.  This should be 100% saturated.  You will have to find a chart detailing various temperatures and pressures to find out the true value for your conditions at any given point in time.

For ORP meters I have no idea.

For regulatory purposes the guidelines for measuring BOD are the same no matter the magnitude of the BOD value.  The dilution water is always prepared the same, the GGA is always at the same concentration, and the temperature and time of incubation remain the same.   If you have some non-regulatory samples you wish to analyze you can do a few things to get low level values.  The first would be to increase the volume of the sample.  After that you could try incubating for a longer period of time.  For both of these options you would need to qualify the data once you generate it so that someone looking at it later would know the differences in the data.  You can try adding proportionally smaller amounts of the GGA standard but there are no guarantees as to how good the results would be.