What is the role of dichloromethane in the ozonolysis / Criegee oxidation?
Sometimes the solvent choice is very important to enhance or decrease reactivity of either of the reactants.

Dichloromethane is a widely used organic solvent. Chemists sometimes say tongue-in-cheek that it can dissolve almost anything organic since it is rather mid-range in polarity. Other than that, it is also non-nucleophilic, non-electrophilic and generally does not take part in many reactions. (There are certain exceptions, however!) This means that it is a standard solvent choice for many reactions and the ozonolysis is no exception. Most reactions require a solvent to allow all reactants to get into close proximity of each other for the reaction to occur at all — not many reactions can also be conducted in solid state.

The standard mechanism for ozonolysis — the Criegee mechanism — is given for solvents such as dichloromethane, hexanes or any others that do not participate in the reaction in any way (that are inert). However, when using methanol or other alcohols the mechanism must be modified as also highlighted by Criegee in the original mechanism publication. I have included the mechanism for ozonolysis both in presence and in absence of methanol in the scheme below.



The resulting peroxy-methyl-hemiacetal is much more stable than the carbonyl oxide and thus various side reactions, e.g. formation of Staudinger ozonides with remote carbonyl groups are inhibited. Thus, in the case of the ozonolysis, adding methanol decreases the reactivity of the ozone in solution. Conduction the reaction in dichloromethane should be considered the standard case — unless one wants to say that it enhanced ozone’s reactivity.

These details can also be found on organic-chemistry.org and this page from Michigan State.