The oil the Ukraine stole was from the Russian pipelines running through the Ukraine, to Europe.
Here is an example of proof of Ukraine theft.
en.wikipedia.org
{...
On 8 June 2010, a Stockholm court of arbitration ruled
Naftohaz of Ukraine must return 12.1 billion cubic metres (430 billion cubic feet) of gas to
RosUkrEnergo, a
Swiss-based company in which Gazprom controls a 50% stake. Russia accused Ukrainian side of diverting gas from pipelines passing through Ukraine in 2009.
[11][12] Several high-ranking Ukrainian officials stated the return "would not be quick".
[13]
...}
The Ukraine committed half a dozens acts of war, so Russian then had no choice.