You have several options:
1 - IDEAL / NO WORK AT ALL
It may be possible to keep a single read only license to your old system
- Pros - easy to look things up, nothing new to learn
- Cons - it is another system, so now things are in two places
2 - QUICKEST / EASIEST
Import Excel sheets of the relevant information into Odoo Spreadsheets via the Documents App
- Pros - get access to what you need without much work to import
- Cons - harder to search for things, not integrated well
3 - MORE WORK
Create a Studio App and import tables of CSV data
- Pros - can use Odoo filtering and search, form and list and kanban views, can link to Customers via Smart buttons
- Cons - takes more time
4 - EVEN MORE WORK
Import Monthly Trial Balances
- Pros - you can run year on year comparisons
- Cons - takes even more time
5 - THE MOST WORK
Import Transactions
- Pros - works as if you were using Odoo the whole time
- Cons - VERY VERY VERY time consuming, needs scripts, very advanced knowledge required, may not have a good return on investment
Resources for going live with Odoo Accounting:
https://drive.google.com/file/d/1QJ_nRLNq1kjqX-oapYNo_3JYBpnFgap2