I have an IR spectrum that has signals for my expected product but also signals from what I believe to be some reactant (an alcohol). Typically, an O-H signal is pretty strong, but this O-H signal has an absorbance of only ~35%. Can I make a conclusion about the relative amount of reactant?
No. Frankly, in organic chemistry, IR is usually just used as support for what functional groups you think should be on your molecule. I only obtained IR spectra because you need to include them with supporting information when you want your paper reviewed.