When I go to buy, Fidelity advises me to purchase on the OTC because the costs of the transaction are lower. I'd have to call Fidelity to fully understand whether a currency conversion also occurs.
The OTC has American depository shares, and you place the bid in US for whatever price you are willing to bid. As for transaction costs, I use TD Ameritrade and every trade is $6.95 whether you are buying 1,000 shares of NIOBF or 1,000 shares of Berkshire Hathaway.