I have a oil tankless heat/ hot water and I thought I was running low on oil so I added some and the heat works on the base boards but the hot water won't get hot. How do I fix this?

How old is the boiler (and tankless)?
Tankless off the boiler gets calcified quickly, not allowing more than a few ounces of water to heat up, and not allowing much heat to get to the water either.

There are de-mineralizing products you can add to the tankless, but they don't really work. You need a new one. You might want to spring for a separate one, propane or NG if you have that. Or a plain old 40 gallon electric water heater.

It's a coil burner and I was just told it might be a 45 dollar part similar to a thermostat in a car.

It's a coil burner and I was just told it might be a 45 dollar part similar to a thermostat in a car.coil burners do not have tanks correct?