OUR rates increase in response to higher soluble organic loadings (BOD5), faster microbial division (low F/M), and following upset conditions when microbes are moving back towards stable population - this is log phase growth.
Another facet of OUR testing includes a potential for a drop in respiration rates due to acute toxicity where previously respiring microbes are inactivated (killed). I have seen this with phenol, cyanide, and tall oil releases in industrial wastewater.
So any large change in OUR rates should be investigated. If you are changing MLSS concentrations, you should standardize the OUR by using the SOUR calculation (divide OUR by MLSS or MLVSS in grams).
Here is a link to the OUR test protocol that I have used for the past 25 years.