I came back from my holiday in Sanlucar with two problems of etymology.
The first referred to the title of this piece; Bonanza.
This is the name of the little fishing port which is just up the coast from the town of Sanlucar, it is so close (we walked up the beach to it in a half hour) that it qualifies as being part of the town and is in fact officially known as Bonanza de Sancucar de Barrameda.

Now we all know that a bonanza is a stroke of luck, or a sudden happy event.
The OED led me back through the Latin Bonus, good, to it meaning good weather.
But no-one one seemed to know how the port got named.
I found the answer in my reliable Encyclopaedia Britannica, 11th edition, all 32 large volumes of which I have managed to carry with me throughout my married life. (Even though they now rest in ignominy on the floor of the attic in France they are soon to be well housed.)

The Spanish government as they began to realise that the conquest of the Americas was a Good Thing (gold and silver being certainly Good Things) founded, as all colonial governments do, a company to take care of the business which they called The South American Company.
This was based in Seville, just up the river from Sanlucar.

To propitiate the Gods or in this case Nuestra Senora, they took out an insurance policy with them and built a church at the port of Sanlucar which they called La Virgin de Bonanza- Our Lady of Good Weather.
The port then was named after the church.

That problem of origin solved leads me neatly to my second, as yet unsolved, etymological mystery.
This is the reason why our local church was known as the church De Nuestra Senora de la O- or the church of Our Lady of the O.

The best (of a bad lot) explanation I can find so far is that at the moment of birth Nuestra Senora said O, and this then became symbolic of the Virgin Birth,
much celebrated in Spain, as is the Immaculate Conception.
My researches continue.