TOTV - Traded on a Trading Venue
Means that the shares were bought through the Stock exchange the Shares are listed.
OTC - Over the Counter
Means that you bought the shares from another Trading212 client or T212 itself effectively.
You will typically see OTC when buying fractions of a share or if the best price available is being offered by another Trading212 Client. For Taxes I think it still counts as the listed stock exchange, but @Team212 can better answer this with certainty.