how to do a SELECT of two different fields of the same table in a single column in Oracle 11g?

Assuming that serialnro and serialmac are of the same type, and only one of the 2 has a value that is not null at a time, then you can use coalesce to return the value that is not a null :

select desc_equipo,
       coalesce(serialnro, serialmac) as serial
  from product

Edition

  

that is, sometimes those who use the system leave a blank space

It's a shame that you have spaces mixed with null to represent a nonexistent value. But if this is the case, you can correct the query by means of an additional trim that converts the empty spaces to null , so that the coalesce works correctly:

select desc_equipo,
       coalesce(trim(serialnro), serialmac) as serial
  from product

I feel that the structure of your table is the wrong one, if all the products have a serial but some are nro and others are mac, then you would have a column for the serial and another column to indicate if it is mro or mac, your columns would be:

SERIAL VARCHAR2
SERIAL_TYPE VARCHAR2

and you could add a constraint to the Serial Type column so that it only accepts the values that you define

ALTER TABLE PRODUCT ADD
CONSTRAINT PRODUCT_SERIAL_TYPE_CHK CHECK (PRODUCT_SERIAL IN ('MAC','NRO'))

This way you would eliminate your problems with blank and null spaces, any doubt I am at your orders.

Greetings!

You can also use:

SELECT desc_equipo, TRIM(NVL(serialnro, ' ') || ' ' || NVL(serialmac, ' ')) AS serial
  FROM producto

In case there are both in a registry, they will appear concatenated.