Mass data transfer

Before I start talking about my excel needs I just want to wish everyone a merry x mas!!

Now lets talk about excel, I need to transfer a large amount data (~2,5 million lines of data) from one spreadsheet to another while making sure the destination file is kept in a certain format.

Spreadsheet "oxno" is the file that needs filling out with data provided to me in spreadsheet "ox"
The value that is the matching criteria will be the ITS account number (Field B in "ox" and field A in "oxno") and the currency (Field B in "oxno" and field G in "ox")
So I want a macro to look in "oxno" and use field the values of field "a" & "b" and search for the them in "ox" in fields "B" & "G". Once it finds a match it should copy the value of field "I" in "ox" and paste into field "n" on "oxno".
The next data to be copied should be what goes into field "O" in "oxno" this data is derived from field "L" or "M" but only when its of numerical value with one non numerical field in it (usually a "/").
Last value that I need copied over is Field "T" on "oxno" this is derived from field "J" or "M".

I realise the potential conflict because two sets of queries will be looking to gather values from field "M" on "ox". That is why I am hoping when we look for value for field "O" in "oxno" we only look for numbers with one non numerical field in it (usually a "/") .

After finishing writing this up, I realise how wordy this has become appologies for that, but hopefully someone will be able to understand what I am looking to achieve and will be able to suggest a solution.

Thanks in advance.

Nowshad