Regarding opening the back, it’s “lefty loose-y and righty tight-y”. But I would only let a watchmaker do it. There is a good chance that the rubber O-ring inside has turned to liquid goop and you might end up with a mess on your hands.
The bracelet is not made by Omega and has little value. The crown is missing its plating and needs to be replaced. The crystal also needs to be replaced.
The movement is likely a cal. 268 or 269, which is one of Omega’s better manual movements, but it likely to require servicing after all these years of not being used. Dial is nice and original.
Someone is going to need to invest $500 or so to get it restored properly. I can see $300 to $400 (US) for this in current condition, but not more.
Sorry to be the bearer of bad news.
gatorcpa