OK, thanks vs OK; thanks

OK, thanks is a comma splice and is the typical form. I would say that the reason it is used over OK; thanks is simply convention. Semicolons are rarely seen in English writing, news articles or conversational writing such as emails. I only ever really use semicolons to separate email addresses.

However, (comma :) having said that, I've noticed a few recent blog posts on the use of semicolons which lead me to believe that semicolons should be used more frequently. The best summary is here:

An Easy Way To Know When (And How) To Use A Semicolon

1. THEY FORM CONNECTIONS BETWEEN SEPARATE SENTENCES.
2. THEY CAN REPLACE COORDINATING CONJUNCTIONS.
3. SEMICOLONS CAN BE USED AS SUPER COMMAS.

So in the cited example and using the "rules", then "OK, thanks" is (perhaps :) the correct usage here because the full sentence is "I agree on your point and thank you for it".