The OSCE did not declare that there is no Russian military in Donbass.
The Chief of the OSCE Special Monitoring Mission in Ukraine told the UN Security Council that: "there has been cases where we have reported in the past people in uniforms, claiming to be from the Russian Federation and other countries as well as mercenaries. SMM has reported observations of vehicles bearing Russian flags, but did not observe the origin. I have to say in order to avoid many misunderstandings, in these reports there has been no reference to militaries". The video of the statement here [2:45:00 - 2:45:47].
This is not the first time OSCE statements are misquoted or taken out of context.
Consistent with recurring pro-Kremlin disinformation narrative that there are no Russian soldiers in Ukraine. For similar cases see here.