BitLocker recovery key page lists OSV and FDV, what's the difference?

OSV is the operating system volume, your C: drive. FDV is a fixed data volume, any other internal drive. At boot it's asking for the OSV one. Don't guess by the label though, the blue recovery screen shows a Key ID, and the page next to each key shows the same ID. Match the first eight characters. Several OSV entries just means the drive was encrypted more than once, the ID is what picks the right one.