Denying the presence of Russian forces in Donbas is a standard pro-Kremlin disinformation narrative. Moreover, saying that the OSCE did not find evidence of Russian participation in Donbas goes against some statements of the former First Deputy Chief Monitor to the OSCE Special Monitoring Mission to Ukraine, Alexander Hug. It is not the first time that such claims are made.
In an interview, delivered in autumn 2018, Hug mentions that the Mission has been recording the instances of Russia's involvement into the war in Donbas. "We have been seeing at least in six instances convoys leaving and entering Ukraine, which we have been documenting, which have been driving towards the unsecure border with the Russian Federation in the middle of the night, on dirt roads, and have been seen at the unsecure border at areas where there is no official border crossing," Hug said.
"The Special Monitoring Mission has also seen tracks that are in close proximity of the unsecure border with the Russian Federation. We have also described in multiple times […] specific types of weapons that we have seen in the areas not controlled by the government, and we have seen and spoken to prisoners taken by the Ukrainian Armed Forces who claimed to have been members of a Russian unit fighting on rotation in Ukraine. All of these facts have been made available and I have been referring to them, including in that interview. These facts speak for themselves," Hug concluded.
See here for more information about Russian forces in Ukraine.