I named your books as shown in the red code:

Sub Sunirone(): Dim wb1 As Workbook, wb2 As Workbook, r As Long, K As String
Set wb1 = Workbooks("Sunirone1.xlsx"): Set wb2 = Workbooks("Sunirone2.xlsx")
With CreateObject("Scripting.Dictionary")
r = 4: Do Until wb2.Sheets("Sheet1").Range("C" & r) = ""
K = Trim(wb2.Sheets("Sheet1").Range("C" & r)): .Item(K) = r
r = r + 1: Loop
r = 4: Do Until wb1.Sheets("Sheet1").Range("B" & r) = ""
K = Trim(wb1.Sheets("Sheet1").Range("B" & r))
            If .Exists(K) Then
wb1.Sheets("Sheet1").Range("C" & r) = "OK"
Else
wb1.Sheets("Sheet1").Range("C" & r) = "Not Found"
            End If
r = r + 1: Loop
End With
End Sub