I honestly have never seen 24VDC valve myself — most sprinkler valves are 24VAC. Thought this sounds ironic: if you use DC to operate AC valves, you can actually use lower voltage (typically 12VDC is sufficient to operate 24VAC valves). This is due to the way AC valves work, and the specifics are explained in this article by me:
Now, regarding 24VDC valves: I wouldn’t recommend powering OSDC with 24VDC because the voltage regulator on the driver board is not really rated for 24VDC — I think it still works for a short period of time but it’s beyond its maximum rated input voltage, so I think it’s a long-term reliability issue. I would suggest that you try to use 12VDC to power OSDC, and then see if it can operate your 24VDC valves. I think there is a good chance it will, because most valves requires a high impulse current to energize and then you can lower the voltage to provide holding current. OSDC is exactly designed to do so: it generates a high impulse voltage and then lower the holding voltage to the input (in this case 12VDC). That said, I’ve never seen a 24VDC valve myself so I cannot say for sure this works. You can give it a try.
Sorry, right after I wrote the above message, I saw you have already tried your valve with 7.5VDC. If it works, then I am pretty confident it should work with OSDC with the 7.5VDC power supply that comes with it.