I suspect it may be a space within the number, for example this will cause the error

SELECT CAST('123 456' AS BIGINT)

If you want to compress this situation into a single number then you could do

CASEWHEN PATINDEX('%[^0-9]%',REPLACE(h.oe_po_no,' ','')) = 0

THEN Cast(REPLACE(h.oe_po_no,' ','') as BigInt)

ELSE 0

END = 0

Otherwise to throw a zero

CASEWHEN PATINDEX('%[^0-9]%',RTRIM(LTRIM(h.oe_po_no))) = 0

THEN Cast(RTRIM(LTRIM(h.oe_po_no)) as BigInt)

ELSE 0

END = 0