1. Since private keys start with 5, you're going to be leaking the first digit of your OTP.
2. Using your OTP more than once can lead to it being compromised. You're lucky that the underlying private key data is random, but you're in trouble if one of your private keys is discovered.
I know the first digit of the OTP is leaked. Is a private key really random appart from the first 5? If not the OTP can be compromised.
I know you should never use a OTP more than once but I think it's ok with private keys has long has no unencrypted private key is leaked.
Edit : After rethinking about what you said Insti. I should generate a different OTP for each private keys. This way I don't have to worry. I also found out that a private key is not completely random. Because of this I will generate the OTP randomly without using another private key.
Nearly every 256-bit number is a valid private key. Specifically, any 256-bit number between 0x1 and 0xFFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFE BAAE DCE6 AF48 A03B BFD2 5E8C D036 4141 is a valid private key.
The range of valid private keys is governed by the secp256k1 ECDSA standard used by Bitcoin.