Schools here can be a bit confusing like that. When our children were in Eton House they didn't have a K2, they had K1 and then Year 1 and although their school year ran from August to June, the cut off dates were Jan-Dec which sounds the same as OFS although their year groups are named differently.
At our current secondary, I think if a child moved from OFS to our school, they'd go from being in Grade 8 to year 7 - basically the same year group but a different name. Some people moving to our school don't like it, because their children feel that they've gone backwards a year, but I think that some schools follow a US style of grading and some follow a UK style of year groups - in the UK now I believe you leave in Year 13 whereas in the US it's Grade 12 (could be the other way around - I still find it confusing myself and my kids are in Secondary

)