Is a stream cipher similar to One-Time Pad using hash secure?
If you are imagining something like 256+ bits of key -> SHA-256 -> one block keystream that you XOR with the plaintext, that is indeed almost the same as one-time pad. However, there is no advantage to doing that, considering you might as well have used the original key directly as a one-time pad.

If, instead you use a password-based hash, a dictionary attack or similar can be successful, and unless the password has more entropy than the output size of the hash, the attacker can at least rule out some possible messages, meaning it lacks perfect secrecy.

Even in the first case, where your key is high entropy, the fact that you run it through a hash function will affect the possible outputs and their frequencies, so a computationally unbounded adversary would gain some information about the contents of the message – some options may be impossible and some may be more probable.

In practice, using a (password-)hash to produce a keystream amounts to a stream cipher, and assuming your key/password has enough entropy and you do not reuse the same key/password for other messages, it is very likely secure. From a confidentiality point of view, that is: stream ciphers (and OTP) are malleable so you want authentication if the integrity of the message matters.

(Insert obligatory recommendation to use standard ciphers in practice.)