you are right, you oscilloscope gnd is connected to earth.
You battery can modeled basically by a floating voltage source of 12V.
Because it is floating, you can connect its minus terminal to the gnd of the oscilloscope.
At the positive terminal of the battery you will get a voltage that is 12V greater than your oscilloscope gnd. If now you want to measure that 12V with your oscilloscope probe, make sure that 12V is not greater than the max input voltage of the probe. This is generally not an issues if you use standard 10x,1Mohm probes.
But pay attention if you plan to connect to the oscilloscope directly using a BNC connector for instance. The Inputs of my Lecroy accept max 5V and are 50 ohm...
Imagine 12V in 50 ohm :
$$ P = \frac{U^2}R $$
$$ P = \frac{12^2}{50} = 2.88W $$
2.88W the be dissipated by the input of the scope...
"Ground" is just a code word which, in this case, refers to the "current return common" circuit node. There is a complete circuit because everything electrical in the car, such as the starter motor, also connects to ground in order to return current to the minus terminal of the battery through the ground. The car's chassis is used for this return network, and so the entire chassis is an extension of the minus terminal of the battery.
During jump-starting, we connect the boosting battery to ground rather than to the dead battery's - terminal for the simple reason that this provides a more direct return path to the good battery which is powering the dead car: the return current does not have to travel through the dead battery's minus terminal hookup cable and then to the jumper cable, but can go directly from the chassis ground to the jumper cable.
A more direct return path allows for better current flow and less voltage drop, like plugging a big appliance directly into an outlet, rather than via an extension cord.
In case you're also wondering why the plus jumper connections are made first, then the minuses. This is because there is no harm done if you leave the minus jumper dangling in the chassis of the car. Anything it accidentally touches is likely to be ground. If you connect both alligator clips on one end before connecting the other end, the other end is now live and you can accidentally touch the clips together to create a short circuit. If you connect the minuses/grounds first and then go to connect one of the pluses, you can create a short circuit, because the opposite side plus is probably dangling and touching something that is grounded.
Best Answer
Yes - just remember that your ground in that case will only be relative to the battery. If you go to connect this to another device (serial interface, etc) you need to link the ground lines so they're a common ground.
So long as it is isolated, you're fine.