The 18K (0.750) hallmark is for the case only, not the movement. I dropped a post in the recent discussion about pocket watches saying it was common to find expensive gold cases with cheaper low-grade movements inside them because the owner was more concerned with "form over function" - and this watch appears to follow that rule!
The movement is possibly the cheapest I've ever seen in an expensive watch - only a very few jewels, the balance wheel is not compensated and cannot be poised, and no precise adjustment for regulation - just a step or so above a "dollar watch". The value here lies in two areas - the gold and jewels in the case, and any family history it might have. As a watch you would probably get more selling it for the value of the gold, especially since gold is near an all-time high and possibly going higher very soon. As an Omega collectible, I cannot tell if the movement is an actual Omega or an inexpensive replacement that a previous owner had put in instead of fixing the original - which was sometimes a much less expensive alternative, especially for a watch that wouldn't see much use.