what would be the best type of oxigenated plant (s) to put in a garden pond?
Answers:
(DISCLAIMER: I'm sorry if you live a temperate country, because the following advice will ONLY work in a tropical country, for something like a koi carp pond, or a large general garden pond.)
I think your main motivation is to provide oxygen for fish in the pond. (It's always useful to have plants anyway, because they are an ideal source of shade and shelter, and are ideal for fish spawning. And that's quite apart from adding aesthetic value.)
I would add three types of plant:
a. Fully submerged;
b. Marginal plants;
c. Free-floaters.
SUBMERGED PLANTS: My first choice would be Vallisneria, an aquarium staple which is almost impossible to kill. Rock-solid performer that will also thrive in your pond. Grows moderately fast.
Hornwort is also good; Cabomba is beautiful but a little fragile - I'd try it later on, once your basic plants are settled in.
MARGINAL PLANTS:
If your pond has a slope, do something nice in the shallows with water iris, rushes, reeds, canna lilies, caladiums, arrowheads, maybe even Wandering Jew or 'money plant' (Jade Pothos - it will eventually spread out to the rest of the pond, so trim it back regularly, but it is very hardy. Even if nothing else can survive in a pond, it will!)
FLOATING PLANTS:
For sheer drama and beauty, you can't beat lotuses and giant water lillies - if you have the space (the 2-foot diameter ones look spectacular in a big pond.) But even the humble water hyacinth is durable, cheap and grows...and grows! It's a good oxygenator and 'water purifier'. Duckweed and frogbit are nice for small ponds.
The link below has a useful little diagram showing you how to 'aquascape' your pond.
Hope it turns out nice.
Other answers:
Ceratophyllum demersum Hornwort
Elodea canadensis Canadian Pondweed
good site is Legalize Weed?