`:` is sufficient indication that you want to bind another variable.

    List<OpportunityHistory> oppHistories = [SELECT Id, Probability, OpportunityID 
        From OpportunityHistory 
        where OpportunityID IN :opportunities];

Or even better:

    SELECT Opportunity.OwnerId, Owner.Name,
        (SELECT Id, OldValue, NewValue FROM Histories WHERE Field = 'Probability')
    FROM Opportunity
    WHERE LastModifiedDate = LAST_N_DAYS:7