Did you synthesize it yourself? Are you sure that you have the compound that you think you have? My first question would be: do you have any other evidence from other analytical techniques that you have indeed the correct compound? Since your question is so general it is very difficult to give a better explanation of why you might not be able to see the OH stretching mode in your IR spectrum.

1st, oh bond can be either ir or raman, or both active.

2nd, it can be over 3500cm－1

3rd, your measurement need to be improved to enhance s/n.

last, as mentioned above by Theo. the molecular structure of yiur synthesized material can be different from your design.

Maybe you have tautomerie, and the more stable form is not OH (enol) but maybe Keto

@Theo Kloprogge yes, i synthesized it. Actually my reactant was Benzaldehyde which have OH group on meta position (i will take IR of this compound (reactant) for confirmation). Reaction happened on -CHO group. Now IR spectrum of my product don't show any peak near 3300-3500 cm-1 for OH group.

Baddar ul ddin Jamali,

if you start from an meta hydroxy benzaldehyde and do a reaction it could also happened that it was a reaction in the phenol OH. Other thing it is that the final structure of de benzaldehyde reacts with the phenol group.

How to do check that the OH of the phenol it is not a salt? and that is why you are not seen the OH signal?

do you still have the C-O signal for the C(aromatic)-O signal?

best