`:` 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 - with one query:

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

This goes to 2 related lists, regular "field history tracking" as well as special thing available only for opportunities that tracks Probability & Stage only. Very useful if you need to have them automapped to the Opportunity.

Yet another format that wastes only 1 query:

    SELECT Opportunity.OwnerId, Opportunity.Owner.Name, Probability, StageName
    FROM OpportunityHistory 
    WHERE Opportunity.LastModifiedDate = LAST_N_DAYS:7
    ORDER BY OpportunityId, CreatedDate