Optical window assemblies are provided. An example apparatus includes a first fixture defining a fluid flow passageway. The example apparatus also includes a second fixture defining an aperture. The second fixture is coupled to the first fixture. A first optical window is disposed in the aperture. The first optical window has a first end and a second end. The first end is to be in contact with fluid in the fluid flow passageway, and a cross-sectional size of the first optical window decreases from the first end toward the second end along a portion of the first optical window.