Did the Germans occupy Kiev or Odessa in WW I?

Neither Odessa nor Kiev were occupied as a result of a military action, see map.

Early in 1918, Ukraine was mostly controlled by the Central Rada and was de-facto independent, although the civil war for control of it was brewing between the bolsheviks and those who preferred independence. The Rada invited the Central Powers (who were all too eager to oblige) and Kiev was occupied by the Germans (1918-03-01) and Odessa by the Austrians (1918-03-14).

This was acknowledged by the Bolsheviks when they signed the Treaty of Brest-Litovsk and renounced all claims for Ukraine.

The Central Powers left in November 1918 after the Armistice.

The Germans and Austrians did occupy Kiev, and they did so as a result from military action, albeit one with less resistance than otherwise expected on that front.

When the Bolshevik delegation tried to stall the Brest-Litovsk peace treaty negotiations the Central Powers resumed offensive aggression in the East against very weak forces. This is known as the "Eleven Days War" or in German as Operation Faustschlag:

German troops in Kiev, March 1918:

On 18 February, the German and Austro-Hungarian forces started a major three-pronged offensive with 53 divisions. The northern force advanced from Pskov towards Narva, the central force pushed towards Smolensk, and the southern force towards Kiev.

The northern force, consisting of 16 divisions, captured the key Daugavpils junction on the first day. This was soon followed by the capture of Pskov and securing Narva on 28 February. The central forces of the 10th Army and XLI corps advanced towards Smolensk. On 21 February Minsk was captured together with the headquarters of the Western Army Group. The Southern forces broke through the remains of the Russian Southwestern Army Group, capturing Zhitomir on 24 February. Kiev was secured on 2 March, one day after the Ukrainian Central Rada troops had arrived there.

Central Powers armies had advanced over 150 miles (240 km) within a week, facing no serious resistance. German troops were now within 100 miles (160 km) of Petrograd, forcing the Soviets to transfer their capital to Moscow.3 The rapid advance was described as a "Railway War" (der Eisenbahnfeldzug) with German soldiers using Russian railways to advance eastward. General Hoffmann wrote in his diary on 22 February:

It is the most comical war I have ever known. We put a handful of infantrymen with machine guns and one gun onto a train and rush them off to the next station; they take it, make prisoners of the Bolsheviks, pick up few more troops, and so on. This proceeding has, at any rate, the charm of novelty.

The Wikipedia entry is as condensed as it gets. If this project needs a tiny bit more of fleshing out some intricate interactions between German war aims, policies and the Ukrainian as well as Russian side you should at least consult:

Fritz Fischer: "Germany's Aims in the First World War", WW Norton: New York, 1967. The most relevant chapters being "13. Germany and the new Russia: The promotion of revolution and attempts to make a separate peace" (especially the part on "The Genesis of the 'Autonomy' Policy" p 375), "17. The objectives of war aims policy, II: Between annexation and self-determination", "20. The elaboration of the Ostraum: The Ukraine, the Crimea, the Don Transcaucasia"

This will get you a feel for the ambitious aims and also attitudes of many officers of the Imperial German Army (the "Reichswehr" was never officially in Ukraine, existing as an organisation with this name only from 1919–1935).

Orest Subtelny: "Ukraine: A History", University of Toronto Press, Toronto, ⁴2009. This will get you a first impression on the internal dynamics of rada/soviet, left wing and nationalist factions, as well as a first glimpse into who founded what and who funded whom. Money, coalitions and other support going in sometimes unexpected directions.

Although I find the memoirs or diaries of characters like Ludendorff, Hoffmann etc. much more revealing for this type of quest, the war diary of Gottfried Rinker who was in Odessa is partly online.