BitPie wallet doesn’t use the standardized account key generation process specified in ADR 0008. Consequently, your Bitpie wallet’s mnemonic phrase will not open the same account in Oasis Wallet.
The preferred way is to create a new wallet with an Oasis Wallet and transfer the tokens to this new address using your BitPie wallet. The cost (i.e. transaction fee) should be negligible.
If your tokens are staked/delegated, then you need to debond them first which will take approximately 14 days. Afterwards, you can transfer them to the new Oasis Wallet address and stake/delegate them via an Oasis Wallet again.
Alternatively however, if you do not want to transfer the tokens over the network, you can export the private key from your BitPie wallet and import it in Oasis Wallet in 2 steps: