Honestly I would say to do it in your DB, get a date (the first or last) next to your week ID (or directly get the start and end date of the week next to the week ID).
In OBIEE your week ID "YYYYWW" is in the best case a number or in worst a varchar: nothing close to a date.
Also keep in mind that a week ID can have different meanings .... When does your week start? Monday or Sunday? When is week 1? For some it's the first Monday of the year, for some others it's the week of the 1st January, sometimes the first days of a year are part of the last week of previous year (depending if 1st Jan is before or after Wednesday) etc.
So I would really suggest to get a real date for the week ID in your data source, at least you can't be wrong about the meaning of week in your context.