Place the butter in a heatproof bowl and soften by beating with a wooden spoon.
Put the measurement water, salt and oregano in a saucepan, bring to the boil and cook for 4–6 minutes until it has reduced a little, then pour it over the butter and stir with a wooden spoon until well blended. Leave to cool.
Knead the cooled butter with your hands, squeezing out any excess water. Drain well, then spoon the butter into a hot, sterilized jar. Seal the jar and store in the refrigerator, or a cool place, for at least 6 weeks before using. Use within 3–4 weeks.