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: We did a cohort analysis to estimate and compare incidence of VTE in users of the main OC preparations , and a nested case - control study to calculate the odds ratios of VTE associated with use of different types of OC , after adjustment for potential confounding factors .
### Output Text:
We did a cohort analysis to estimate and compare incidence of VTE in users of the main <span class="chemical">OC</span> preparations , and a nested case - control study to calculate the odds ratios of VTE associated with use of different types of <span class="chemical">OC</span> , after adjustment for potential confounding factors . 