New Tax Code OT W1, Very Confused, threshold?

Can't tell you exactly what your situation might be, but the HMRC's definitions for those codes can be found here:

http://www.hmrc.gov.uk/incometax/codes-basics.htm

NT - Is used when no tax is to be taken from your income or pension

Tax code ‘0T’ means your allowances have been used up or reduced to nil and your income is taxed at the relevant tax rates.

I know that when you have W1 or M1 after your code it means that your personal allowance is spread out proportionally over the year, so maybe they can see you will go over your personal allowance, so they are taking tax now to spread your payment out.

any chance you did extra hours? i've been in situations as a student where for some mistake, about six weeks went through as one go, so i went over the weekly threshold for tax. very annoying, but could be claimed back.

although i don't understand why you would pay tax without NI. that's odd!

It sounds as though your employer regularly operates code NT against your wages. That would explain why you normally pay no tax until the latter part of the tax year. Then, once you have exceeded the personal allowance for the year, currently £6475 your employer taxes you on everything over that.
That is not how the system is supposed to work.
You really should be on code 647L.
Then, in the first week of the tax year you would be allowed to earn (6475/52) £124.50 free of tax and you would pay 20% tax on anything over that.
In the second week of the tax year your wages for week 1 and week 2 are added together and you are allowed £249 tax free and so on.
The problem you now have is that you total pay to date is £5,452 but the end of December roughly equates to week 39. Therefore at that point you were allowed to have (39/52 *6475) £4,856.25 free of tax and therefore should have paid about £119.20 tax.
Turning now to your code numbers on your payslips as others have already said you really should not be on code NT.
Code 647L W1 is not really correct. W1 stands for week 1 basis and we can come back to that if we need to.
Code 0T W1 is wholly inappropriate. It means you pay 20% tax on everything.
If you check Wage slip 3, with gross pay of £79.70 the correct tax on code 0T W1 is (79.70*20%) £15.94. So, even using that code your employer has taken too much.
So what is going on? 3 consecutive weeks with 3 different codes. With the best will in the world, HMRC really don't move that fast.
Whilst it is possible that HMRC have been involved I rather feel that the problem is more likely to be with your employer.
There are processes where employers do set codes for new starters but, normally speaking, they should not alter codes for current staff.
However, in your particular circumstances I find it easy to believe that your employer has found out or been advised that they have wrongly been using code NT in your case and gone into panic mode to recover the tax they should have already deducted.
I would therefore suggest you have a word with the payroll people to try to understand what has happened at their end.

It looks to me like your payroll lady has got it wrong all year and is now trying to retrieve the situation without having to admit she's got it wrong. She's tried it one way & it didn't work, tried it another way & it still wasn't working & finally tried it in a way which would claw back the tax due & hopes no-one notices. About 6 or 7 weeks down the line she should be able to put you on the correct code 647L. But, as has already been said, a quick call to HMRC to check what they have as your code is a good idea.