Yes, your hint is not correct. Try:
SELECT /*+ INDEX(A GRID_MASTER_LVL_IDX)*/ a.grid_lvl2_code, b.grid_lvl3_code
FROM grid_county_centroid B,grid_master A
WHERE sdo_relate(B.centroid,A.sob_geometry, 'mask=anyinteract querytype=join') = 'TRUE'
and a.grid_lvl = 2;