You can obtain the oldest Open Transaction in the following way:
-- Create temporary table for results.
CREATE TABLE #OpenTrans (
CurrTrans VARCHAR(25),
CurrTransDet SQL_VARIANT
);
-- Insert info in temp Table
INSERT INTO #OpenTrans
EXEC ('DBCC OPENTRAN WITH TABLERESULTS, NO_INFOMSGS');
-- Display results.
SELECT * FROM #OpenTrans;