How to Track field history on Opportunity Line Items?

Hi,
You can do it using Custom Object and Trigger not with the Standard History Tracking.
Step 1:
Create a custom object(History_Tracking__c) to store your object history.
Step 2:
Create a Trigger on object where you want to track fields.
Below is the sample code:

trigger trackOppProduct on OpportunityLineItem (after insert, after update) {
List<History_Tracking__c> hisTrackList = new List<History_Tracking__c>();
//
// Iterate through the OpportunityLineItems and create the History Tracking Record.
//
for (Integer i=0; i < Trigger.new.size(); i++) {

OpportunityLineItem newCM = Trigger.new[i];
if(Trigger.isInsert) {
hisTrackList.add(new History_Tracking__c(OpportunityLineItem_Record_Id__c = newCM.Id,
Object_Name__c = 'OpportunityLineItem',
Field_Name__c = 'Status',
Previous_Value__c = '',
Current_Value__c = newCM.Status,
Modified_Date_Time__c = System.now(),
Modified_By__c = UserInfo.getUserId()
));
} else if(Trigger.old[i].Status != newCM.Status){
OpportunityLineItem oldCM = Trigger.old[i];
hisTrackList.add(new History_Tracking__c(OpportunityLineItem_Record_Id__c = newCM.Id,
Object_Name__c = 'OpportunityLineItem',
Field_Name__c = 'Status',
Previous_Value__c = oldCM.Status,
Current_Value__c = newCM.Status,
Modified_Date_Time__c = System.now(),
Modified_By__c = UserInfo.getUserId()
));
}
}
if(!hisTrackList.isEmpty()) {
insert hisTrackList;
}
}