If you think it will be faster using a seek then just force the query to do a seek and see if it's faster
select ol.ord_no, pl.ID
from oeordlin_wv ol
inner join wsPKGLin pl WITH(FORCESEEK (lnc_ordno_seqno_includes(Ord_no, Seq_no)) )
ON ol.Ord_no = pl.ord_no
AND ol.line_seq_no = pl.Seq_no
where ol.ord_type = 'O'
option (recompile, maxdop 1)