Personally, I don't think it's . . .
an open oil tube but . . . .
Normally, the 'tick-tick' occurs AFTER the car has been started for a few minutes, not at first start. But then again. . .
So the sound does not go away and it gets worse as the car gets hotter probably because the oil 'thins' out abit.
It easy to check: once the valve cover is off, you can visually inspect in just a few minutes. And if one is bad, it's easy to change.
Re oil pressure: you SHOULD notice a drop in oil pressure if one or more tubes is open. If your oil pressure is still high (say around 2 bar when hot), it probably is not a open tube.
But it COULD be a weak lifter that can't hold oil overnight and when started, it takes a few seconds to 'pump-up'. This is the most likely scenario based on what you wrote. These are a pain to replace because you first have to find the bad one (or ones). Then replacing it is not the best job in the world: remove camshaft, chain, etc.
BTW, you should be able to locate the side it's on pretty easily, if not, then more info that it's not a lifter.
Regards . . . . JimF
'94 S500 Cpe
Visit my Mercedes Web Page