How to make conditional join when containing Nulls in some columns use for join
First of all, your description does not match the ON condition. Your description would fit an ON like this:

Select ...
from ...
     meas_data  m
     left outer join  tlea  t
       on  m.dset_cob_dt = t.tlea_cob_dt
       and m.tlsn_trd_id = t.tlsn_trd_id
       and (
                 m.tlsn_vrsn_num = t.tlsn_vrsn_num
             and m.tlsn_leg_id   = t.tlsn_leg_id
           or    
                 m.tlsn_vrsn_num = t.tlsn_vrsn_num
             and m.tlsn_leg_id is null
           or                                          
                 m.tlsn_vrsn_num is null
           )


As for efficiency, I'd suggest a composite index on the 4 columns, on each table (but I'm nowhere near a Oracle expert, so take this suggestion with caution).