Understanding thermocouple transmitter wiring
Start with the symbol for twisted wires: -

Symbol for cables and electrical wiring
It's not identical to what you have but, as far as I can tell, it's saying that the two wires from Vo and OPG are a twisted pair. Furthermore, the dotted line around the pair implies that a screen or shield should be used. So, you need to use shielded twisted pair cable.
I can't tell you what OPG exactly stands for but, for a balanced transmission of a signal from the device, the same impedance should be present in series (and internal) with Vo and OPG and so, you should not consider connecting OPG to anything else.
Here's another example of the same symbol used taken from this web page: -

See also this stack exchange Q and A entitled &quot;What symbol is this?&quot;.