1.Can you combine an oil fired boiler and a natural gas water heater into the same flue?
I also would like code references if you have them.
2. Is oil a solid fuel?
Oil gas can vent to same chimney 03 irc 2427.5.6.2 …03 nfpa 211 9.8.3
no common connection with solid fuel chimney 03 irc 1801.12 …03 nfpa 9.7.12
Oil might be a solid fuel when it is frozen Ricky, wood, coal, and Uranium are solid fuels. 
Thanks for the help I will look these up. I called it out wrong, its not something we see everyday