You can use this code by using the List of Selectoption in your visualforce page directly. See the modified code below.
<apex:page standardController="Opportunity" extensions="Sample">
<apex:form id="form" >
<apex:outputLabel >Channel Group:</apex:outputLabel>
<apex:selectList id="countries" value="{!Opportunity.Channel_Group__c}" size="1">
<apex:actionSupport event="onchange" action="{!setValue}" rerender="picklist"/>
<apex:selectOptions value="{!ChannelGroup}"/>
</apex:selectList>
<apex:outputpanel id="picklist">
<apex:outputLabel >Channel Group:</apex:outputLabel>
<apex:selectList id="countriess" value="{!Opportunity.Channel_Type__c}" size="1">
<apex:selectOptions value="{!channelTypeOptions}"/>
</apex:selectList>
</apex:outputpanel>
</apex:form>
</apex:page>
Controller
public class Sample{
Opportunity opp;
public List<SelectOption> channelTypeOptions{get;set}
public Sample(ApexPages.StandardController controller) {
opp = (Opportunity) controller.getRecord();
getChannelType();
}
public List<SelectOption> getChannelGroup()
{
List<SelectOption> channelGroupOptions = new List<SelectOption>();
Schema.DescribeFieldResult fieldResult = Opportunity.Channel_Group__c.getDescribe();
List<Schema.PicklistEntry> channelGroupValue = fieldResult.getPicklistValues();
for( Schema.PicklistEntry cg : channelGroupValue)
{
channelGroupOptions.add(new SelectOption(cg.getLabel(), cg.getValue()));
}
return channelGroupOptions;
}
public List<SelectOption> getChannelType()
{
channelTypeOptions = new List<SelectOption>();
Schema.DescribeFieldResult fieldResult = Opportunity.Channel_Type__c.getDescribe();
List<Schema.PicklistEntry> channelTypeValue = fieldResult.getPicklistValues();
for( Schema.PicklistEntry ct : channelTypeValue)
{
channelTypeOptions.add(new SelectOption(ct.getLabel(), ct.getValue()));
}
return channelTypeOptions;
}
public void setValue(){
if(opp.Channel_Group__c == 'Direct'){
channelTypeOptions = new List<SelectOption>();
channelTypeOptions.add(new SelectOption('Outside Sales','Outside Sales'));
}
}
}
