Perhaps another approach would be to include antioxidant excipients to protect the protein. Cysteine and methionine residues are prone to oxidation. Cysteine can be protected by including a reducing agent, such as dithiothreitol. Another way to protect your protein from oxidation is to include EDTA, which binds up trace metal ions that can catalyze oxidation.

You may want to add dithiothreitol or TCEP (1 mM) to check if the reduced form of your protein binds your antibody.

For the oxidizing assay, low concentrations of H2O2 or copper should do it, but since your protein maybe oxidizes itself, I would rather add TCEP and check if this reduced form is able to bind your AB.

HTH

There is a method called COFRADIC for the analysis of oxidized vs non-oxidized peptides (with methionines in this case) by mass spectrometry. Maybe this paper can help where they used H2O2 for oxidizing proteins: Article Redox Proteomics of Protein-bound Methionine Oxidation

Mass spectrometry can be used to analyze oxidized vs non-oxidized peptides (in this example, methionines) using a technique called COFRADIC. Perhaps this publication, which describes how they oxidized proteins using H2O2, can be helpful: Article: Redox Proteomics of Methionine Oxidation in Protein.

Thank you Adam B Shapiro , Hüseyin Besir and Jorgaq Pata for your suggestions. They are helpful and I will move forward using your suggestions.