SQL. Come fare riferimento a una chiave primaria composita Oracle?

 CONSTRAINT fk_column
 FOREIGN KEY (column1, column2, ... column_n)
 REFERENCES parent_table (column1, column2, ... column_n)

nel tuo caso

create table Visit_Treat (
TreatCode CHAR(6) constraint cTreatCodeFK references Treatment(TreatCode),
SlotNum NUMBER(2),
DateVisit DATE,
constraint cVisitTreatPK primary key (SlotNum, TreatCode, DateVisit),
constraint fk_slotnumDatevisit FOREIGN KEY(SlotNum,DateVisit) 
references Visit(SlotNum,DateVisit)
);