F.N.B. Corp., among Pittsburgh’s largest financial institutions, Tuesday completed its merger with BCSB Bancorp Inc., parent company of Baltimore County Savings Bank, last Saturday. The transaction positions First National Bank, the retail arm of F.N.B. (NYSE:FNB), among the 10 largest banks in Baltimore as ranked by market share, and builds its Maryland presence to 24 branches. It was also F.N.B.’s third acquisition in 10 months. F.N.B., based in Hermitage, Pa., about 60 miles north of Pittsburgh,…