How to set the OptionSet value in Plugin

Entity opp = new Entity("opportunity");
opp ["name"] = "Test Opportunity";      // string
opp ["estimatedclosedate"] = new DateTime(2015, 12, 1);              // DateTime
opp ["discountpercentage"] = 0.5;                                   // decimal
opp ["closeprobability"] = 82;                                       // int
opp ["isrevenuesystemcalculated"] = false;                           // bool 

Here is how we would set special field values in Dynamics CRM 2011. 
These include OptionSetValue, Money (Currency), and EntityReference (Lookup).
Entity opp = new Entity("opportunity");
opp["opportunityratingcode"] = new OptionSetValue(2);               // OptionSetValue
opp["estimatedvalue"] = new Money(700.2M);                          // Money
opp["campaignid"] = new EntityReference("campaign", campaignId);