Using very simple words, when was interracial marriage legalized in Ohio?

That sentence does not make any sense. It appears to be asking what state repealed its anti-miscegenation laws before California did so in 1948, or to be stating that some state (Ohio?) did so. Actually, California did not repeal the law in 1948. It was in that year that the California Supreme Court found that such laws violated the U.S. Constitution. (SCOTUS agreed 19 years later.) The statute itself was not repealed until 1959.

If the question means to ask (or state) which state eliminated its anti-miscegenation laws by any means before California did so, the answer to that is "none," because California was the first state with such a law to do so. My quick research indicates that Ohio never had an anti-miscegenation law, so interracial marriage was always legal there, and no law existed to be repealed.

ADD: I see your comment to "T." Since Ohio did not have such a law, your folks could get married there in 1900. However, you should be aware that many states would not RECOGNIZE such a marriage, so if your protagonists leave Ohio you will have to check as to the status of their marriage in other states.

Where did they make it OK to marry someone of another race in 1948? Simple enough for you? How old are you anyway?

Simple as it gets: It doesn't matter.