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From Wikipedia:

The ozone depletion potential (ODP) of a chemical compound is the relative amount of degradation to the ozone layer it can cause, with trichlorofluoromethane (R-11 or CFC-11) being fixed at an ODP of 1.0.

What units are used to measure 'degradation to the ozone layer'?

Naively, one may think that if an ODP of $1.0$ corresponds to depletion of $x$ megatonnes of $\text{O}_3$ per megatonne of the compound produced, then a compound with an ODP of $a$ will causes depletion of $ax$ megatonnes of $\text{O}_3$ per megatonne of the compound produced.

Is this correct? If so, what is the value of $x$? If not, are the different values for ODP of each compound computed?

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    $\begingroup$ This question would have better appreciated if posted on chemistrystackexchange. $\endgroup$
    – tensorman666
    Commented Jul 7 at 13:50

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From a freely-available NOAA assessment (Scientific Assessment of Ozone Depletion; 1989, chapter 4 ["Halocarbon Ozone Depletion and Global Warming Potentials"]), page 424:

Ozone Depletion Potential (ODP) has traditionally been defined as the steady-state ozone reduction calculated for each unit mass of a gas emitted per year (as a continuous release) into the atmosphere relative to that for a unit mass emission of CFC-11.

So, if you emit no chemicals, the amount of ozone in the atmosphere is its natural level $L_0$. If you emit a chemical $C$ that depletes ozone, the ozone will equilibrate at a new level $L(C)$. The quantity that is proportional to the ODP of $C$ is $L_0 - L(C)$, with an ODP of 1.0 arbitrarily assigned to CFC-11.

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