Rinse the octopus under cold, running water and place in pot dripping wet (you don't want to dry it off- this is the moisture it will cook in).
Add the vinegar and garlic and cover the pot.
Bring to the boil over low heat.
It is tender when pierced easily with a fork (this can vary from octopus to octopus, so time is not a good indicator of doneness in this case).
When cool enough to handle, strip off suckers and reddish-purple membrane on flesh (these are delicious, but the octopus is more presentable without them- enjoy them by yourself!).
Cut the octopus into small tidbits.
And arrange on a platter, drizzled with olive oil and vinegar and sprinkled with oregano.
If you are going to serve the octopus another day, pack it in a jar or small tupperware bowl, cover with olive oil, top with 3-4 tblsps of vinegar and 1 tblsp oregano.
Give a stir.
Cover and refrigerate.