Geosciences? Which part and where you thinking of going?
With a Geophysics degree you will have a very good chance over a standard geology grad in the O&G industry, they like them a lot because they are rather rare (probably due to the maths). Doesn't mean you won't find it very difficult to get a job though, but then in 3-4 years time the industry may have picked up a lot.
Either way don't bother going for an MSci, unless you want to get into research (PhD etc) or just spend an extra year in industry. Don't do one thinking it will open a lot more doors to you than a BSc (best bet is to go to careers fairs for the industry you want to get into and see what the representatives say, generally it's that they think they are generally pointless).