@Arnaud **************
the OA visa is a year long mulit-entry type visa, it allows unlimited entries for the validity of the visa itself
Each entry gets you stamped in for a year OR as long as your valid mandatory medical insurance is valid for (up to a year)
IF just before the VISA itself expires you exit/re-enter the country (by air or land doesn't matter) and you have a new year of the mandatory medical insurance you will get a new entry stamp for a year (or as long as your insurance is good for up to a year)
That's how most people on an OA visa get two years of stay out of a visa that is valid for one year