As it my wont I went for an overly complicated array formula:

=SUM(INDEX(IF(ISNUMBER(FIND(LEFT(B2:B48,1),"OW")),1,0),0)*(IF(ROW(A2:A48)-1=IFERROR(MATCH(A2:A48,INDEX(IF(ISNUMBER(FIND(LEFT(B2:B48,1),"OW")),A2:A48),0),0),0),1,0)))

Confirmed with Ctrl-Shift-Enter, naturally.