Outdoor D.I.Y. Ovens

Building a clay oven - the basics
A clay oven is any hollow, dome-shaped structure constructed out of clay, clay and sand or clay and straw, used for the purpose of baking and roasting food. It has a brick floor and usually a chimney. read more

The 1-hour Brick Oven
This is the brick oven for those who wake up Saturday morning and think “It would be nice to have a brick-oven calzone for dinner”. Nothing is set in mortar; nothing is going to have to be jack-hammered out of your backyard if you change your mind; and it really works! What it lacks in aesthetics it more than makes up for in (nearly) instant gratification. The longest part of the process is waiting for the fire to heat things up. read more.

Wood-fired Earth Oven
You can build this easy outdoor oven that works as well as a custom-built masonry or ceramic model. Cost? Next to nothing. Material?
The earth under your feet – one of the best building materials on the planet – and the skills you need are just the ones you were born with. Here’s how to do it.

Build Your Own $20 Outdoor Cob Oven for Great Bread and Pizza.
The linked article is all about making a cob oven. The construction details have been trimmed back a bit however this article should still give you a reasonable idea of materials and the building process for making your own inexpensive outdoor cob oven.

Masonry Ovens (Wikipedia)
A masonry oven, colloquially known as a brick oven or stone oven, is an oven consisting of a baking chamber made of fireproof brick, concrete, stone, or clay. Read the article.