The biggest barrier to the OK engine is that it needs to be promoted by a non-commercial promoter. For the big euro races thats the CIK-FIA. For the UK its the MSA, France FFSA, Germany ADAC.
The US is almost completely manufacturer series, IAME, Vortex, Rotax. They have no reason to adopt a multi-manufacturer class. Although one of them could do what the UK did and tender it out to one manufacturer (UK chose Vortex but SKUSA would chose IAME). But it would be a risk and why bother. If they don’t run KZ (except for Supernats) which is monumentally more popular world wide why risk it on OK.