Padding oracle attack, encrypting your own message
I'm assuming you mean CBC mode, and let's say PKCS7 padding. I haven't heard of this attack before, and can't really find any sources (please link if you have one! How would an attacker end up with the $\operatorname{Dec}(C_i)$'s?), but here's one way to generate a valid ciphertext.

If the attacker is okay with 15 garbage bytes at the end of his message, he could do the following:

Send $IV = C_{i-1}$ and a random ciphertext block $C_r$ to the oracle a bunch of times. For a little more than 1 in 256 times, $C_r$ will end up having valid padding (e.g. 0x01 for PKCS7). The attacker can simply add such a block $C_r$ to the end of his ciphertext ($C_1, \dotsc, C_{i-1}$) to get a valid encrypted message, albeit with (usually) 15 garbage bytes at the end of the decrypted, de-padded message.