In what circumstances do scientists use observational research and opinion based research?

They don't use opinion based research.

It is always objective.

They utilize and make use of the scientific method in order to have clear basis and evidence for their investigations. Research method is always used to answer every scientific inquiry and in gaining evidential data or knowledge. The scientific method has the following process or at least undergoes the process of

1. Observation

2. Hypothesis

3. Experimentation

4. Interpretation of data

5. Evaluating the data

6. Passing and recording the data

These steps are crucial and the empirical data that these scientists obtain are very important to keep that is why research paper, thesis and dissertations exists.