Not sure is there any limit to transfer her SA?
Yes, the Full Retirement Sum. And you should do that -- it makes complete sense.
Is better to do it on 01Jan2019? or end Dec2018?
Yes.
You should transfer the FRS now (December, 2018) then fill any remaining gap to the new 2019 FRS in January, 2019.
If I do it in Dec2018, will it affect my OA interest for the year 2018?
Yes, but she gets one more month of higher interest. SA interest is higher than OA interest, so earlier is better.
As yoongf mentioned, the CPF Annual Limit is, well, annual. If you're going to make an "all three" top-up to either/both your and your wife's accounts, then you need to get that top-up done using CPF Form VC/1 (or electronic equivalent) a few business days before the end of the calendar year. Otherwise the 2018 CPF Annual Limit is unused and never can be used.
The order matters, though, for your wife's accounts. I think I would do it this way for her:
1. OA to SA transfer this month, to zoom her SA up to the 2018 FRS.
2. "All three" top up this month ($37,740).
3. In January, if there's any gap remaining to the new 2019 FRS (probably not), transfer a few more OA dollars into her SA.
4. "All three" top up in January ($37,740), assuming she's not going to have any other CPF contributions, such as compulsory contributions.
You can shove another $37,740 for you this month and $37,740 for you in January, too. So that's up to $150,960 of cash you can stash between the two of you. You won't have much at all land in MediSave, but she'll stock up her MediSave rather nicely.
....Or you can contribute directly into her MediSave Account, but that must fit within the same $37,740 limit. MA earns higher interest than OA, but it's also more restricted (only for qualified medical expenses). But once her MA fills up (reaches the Basic Healthcare Sum), the "piggybank" aspects of CPF work particularly well. Between this month and next month, you
could jack your wife's MA all the way up to the 2019 BHS -- you have enough CPF Annual Limit to work with to make that happen. So it'd be like this if you want to do that (assuming her MA is currently $0):
This month: $37,740 into her MA
Next month: $19,460 into her MA, $18,280 "all three" top-up
That combination pushes her MA to $57,200, the Basic Healthcare Sum for 2019, and then pushes her SA even higher (because her SA gets both the MA and SA portions of the "all three" top-up) and starts filling her OA somewhat.
On edit: I think I can improve on this.... For your wife, do this, in this order:
1. Make the "all three" top-up first, this month ($37,740);
2. Transfer
her OA to her SA this month;
3. Transfer your OA to her SA, up to the 2018 FRS, this month;
4. Next month, transfer your OA to her SA, up to the new 2019 FRS;
5. Then top up her MA to the Basic Healthcare Sum (or $37,740, whichever is lower);
6. Finally, if there's anything remaining within the 2019 CPF Annual Limit, add an "all three" cash top-up to her accounts.
If you do it that way then I think you leave behind more in your OA (earning 2.5%) and have just as much in her accounts, so that works a little better all around. Check my math, though, please.
