The Omas Ogiva is an excellent pen. It's a great size, very comfortable in the hand, lightweight, and some Omas nibs can blow your socks off, time and time again. Unfortunately, the demise of Omas means that these pens are not only difficult to acquire but they are difficult to troubleshoot when things go wrong. This is a short post that might help if your Ogiva is leaking or if the piston is stiff/stuck, information which I had trouble finding online. Note that I'm an economist and not an expert in pen repair, and I figured this out by fiddling, so exercise caution!
I've had two leaks and both manifest in the same ways: after some light movement, I'd find the caps would have an unusual amount of ink – a bit beyond your normal nib creep – and that sometimes I'd find the section would be a bit inky. It's never been frequent, but after a while I noticed the ink capacity was poor. I'd fill up a pen, use it lightly for a couple of weeks, and find the pen almost empty.
The nib itself can't be removed without a special tool, similar to the tool for a Montblanc 146, but it turns out the section unscrews. The threads are underneath the cap threads (quite a nice design touch) and it seems that these are sealed at the factory. My guess is that the leak is caused by a sealant failure; it breaks down and allows air and ink to travel into the section threads, and on to the cap.
My solution has been to re-seal these threads using silicone grease from a Twsbi pen. Dabbing a bit of grease onto the threads, then screwing the nib unit back into the section (tightly but not excessively) seems to have done the trick with both my Ogivas (purple and teal Albas). I did a little research but haven't found anything to suggest that cotton resin and silicone grease would be a problematic combination, so I'm hoping it's safe.
While the section is unscrewed, it's possible to get at the piston if you want to perform some maintenance. The piston knob unscrews so that the seal is sticking out from the end of the section. If you keep going, the blind cap will detach and this allows the piston shaft to be removed from the barrel entirely.
Reinserting the piston shaft is a little tricky. In the middle of the barrel is a divider between the ink chamber and the piston chamber. There's a gap which is just large enough for the shaft, with a notch on one side. There are grooves on the sides of the piston knob, and the larger of these needs to line up with the notch.
My advice is to insert the shaft at the section end, line it up with the notch, and then grease the seal. This way, you won't get silicone grease on your fingers while attempting to get the shaft in the right way. It's a bit fiddly. It's also a bit fiddly trying to reattach the blind cap so that the piston works properly. (If you have any trouble, try watching Brian Goulet's Twsbi 580 disassembly video.) Don't worry if the piston knob unscrews once the piston is fully extended – this is normal. Once the section is reattached, the piston won't unscrew that far and there's no way for the blind cap to detach. It's a nice, simple design.
Once you've got the piston operating, you can grease up the section threads and screw it back in. If everything has gone right, your Ogiva should be greased up and working properly again, without leaks or a stiff piston. Hopefully this information is as useful to you as it would've been to me two hours ago!