How to prevent blobs under the microscope?
At least some of the blobs that you have appear to be air. Seating the coverslip carefully will help to prevent air-bubbles from forming under the coverslip. Lay one edge against the fluid and slowly ease it down.



You mention using an oil immersion lens. It is possible that water has come into contact with your lens. Oil finding its way beneath the coverslip could also introduce refractory blobs as well.