my solution

gistfile1.fs

(*
   Project Chadwick #2 - Calc Top 5 All Time OBP for San Fransisco Giants
   OBP = (H + BB + HBP) / (AB + BB + HBP + SF)
   0        1         2        3      4     5      6    7 8         9 10 11 12      13      14 15  16 17 18 19 20  21  22 23 24   25
   LastName,FirstName,lahmanID,yearID,stint,teamID,lgID,g,g_batting,ab,r, h,doubles,triples,hr,rbi,sb,cs,bb,so,ibb,hbp,sh,sf,gidp,g_old
 *)
open System.IO
open System.Collections.Generic

// Batter Record
type Batter = { Last : string; First : string; Season : int; AB : float; OBP : float }
let all_obps = new List<Batter>(5)
all_obps.AddRange([| { Last = ""; First = ""; Season = 0; AB = 0.0; OBP = 0.0 };
                     { Last = ""; First = ""; Season = 0; AB = 0.0; OBP = 0.0 };
                     { Last = ""; First = ""; Season = 0; AB = 0.0; OBP = 0.0 };
                     { Last = ""; First = ""; Season = 0; AB = 0.0; OBP = 0.0 };
                     { Last = ""; First = ""; Season = 0; AB = 0.0; OBP = 0.0 } |])

// Calculates the OBP
let obp h bb hbp ab sf = 
  let denom : float = ab * 1.0 + bb + hbp + sf
  
  if denom > 0.0 then
    (h + bb + hbp) / denom
  else
    0.0

// Add the entry to the top 5 if its in the range
let rec check_top_5 entry index =
  if entry.OBP > all_obps.[index].OBP then
    all_obps.Insert(index, entry)
  else
    let new_item_index = index + 1
    if new_item_index < 5 then
      check_top_5 entry new_item_index

let create_batters stats =
  let cols = stats.ToString().Split(',') |> Seq.toList
  // I'm doing this for clarity if I was worried about lines of code I would just use the 
  // the conversion when I call the obp function
  let ab  = float (cols.Item 9)
  let h   = float (cols.Item 11)
  let bb  = float (cols.Item 18)
  let hbp = float (cols.Item 21)
  let sf  = float (cols.Item 23)
  let season = int (cols.Item 3)

  if ab >= 200.0 then
    let b = { Last = cols.Item 0;  First = cols.Item 1; Season = season; 
              AB = ab; OBP = (obp h bb hbp ab sf) }
    check_top_5 b 0

let print_batter batter =
  printfn "%0.3f in %d by %s %s" batter.OBP batter.Season batter.First batter.Last 

// File.ReadAllLines returns a string[] but to use the calc_obp we need a list<string>
// the |> Seq.toList will convert the string[] into a list<string>
let stats = File.ReadAllLines(@".\data\sf_giants_batting.csv") |> Seq.toList
List.map create_batters stats.Tail

// Display the results
printfn "Top 5 Seasons On Base Percentages since the Giants moved to San Fransisco"
printfn "========================================================================="
List.map print_batter (Seq.take 5 all_obps |> Seq.toList)