한방 쿼리로는 어려울 듯 하네요.
절차적으로 풀어야 할 듯 합니다.
PL/SQL 로 출력해 봤습니다.

DECLARE
    CURSOR c0 IS
    SELECT DISTINCT seq1 FROM test ORDER BY seq1;
    v_seq2 NUMBER := 0;
    v_flag VARCHAR2(3);
BEGIN
    FOR c1 IN c0
    LOOP
        FOR c2 IN (SELECT seq1, seq2
                        , ROW_NUMBER()
                          OVER(ORDER BY ABS(seq2 - v_seq2), seq2) rn
                     FROM test
                    WHERE seq1 = c1.seq1
                    ORDER BY seq2
                   )
        LOOP
            IF c2.rn = 1 THEN
                v_seq2 := c2.seq2;
                v_flag := 'OK';
            ELSE
                v_flag := 'DEL';
            END IF;
            dbms_output.put_line( LPAD(c2.seq1, 5, ' ')
                               || LPAD(c2.seq2, 5, ' ')
                               || '  ' || v_flag
                                );
        END LOOP;
    END LOOP;
END;
/