In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: INTERPRETATION : The previously reported increase in odds ratio associated with third - generation OCs when compared with second - generation products is likely to have been the result of residual confounding by age .
### Output Text:
INTERPRETATION : The previously reported increase in odds ratio associated with third - generation <span class="chemical">OCs</span> when compared with second - generation products is likely to have been the result of residual confounding by age . 