Is Old Persian Easy?
I'd say Old Persian is pretty easy. The reason is that it's a consciously constructed language, at least the written form that survives as inscriptions on monoliths that are supposed to be read by any old peasant from 100 yards away. It's also very repetitive. In between new statements, there are about a half dozen costructions of the form, "Adam Kuruš xšayaθiya xšayaθiya vazraka xšayaθiya xšayaθiyanam xšayaθiya Parsaiy dahyunam." ("I am Cyrus the king, great king, king of kings, king in Persia and the other lands.")
It's true that the Zoroastrian canon is in Avestan, but Old Persian language and culture is thoroughly Zoroastrianized. E.g., the following formula appears about two dozen times in the second column of the inscription of Darius at Behistan: "Auramazdaiy upastam abara vašna Auramazdaha kara hya mana avam karam tyam hamiçiyam aja vasiy." ("Ahura Mazda bore me aid, by the grace of Ahura Mazda the army that was mine smote greatly the army that was rebellious.") In fact, Darius is a highly Zoroastrian name. The OP form of it is Darayavauš. The š is just a nominative ending, and the stem is Darayavahu-; in the inscriptions, the h falls off simply because there was no OP cuneiform glyph for "hu," although there was a "ha," which is why the h reappears in the genitive Darayavahauš. (Ditto for the reason that Ahura Mazda is Auramazda in the inscriptions, and presumably Ormuzd in Herodotus.) The point of this is that "vahu" is a northeast Iranian, i.e. Avestan word for "the good," (it's related, I believe, to a Sanskrit word for existence), that got transmitted into OP as a Zoroastrian idea.
Oh yeah, the relationship between OP and Avestan: OP was the language of southwest Iran, Avestan of the northeast. The differences are dialectal. E.g., a lot of s sounds in Avestan become θ in OP (they then became s again in modern Persian because of other dialectal influences, including Arabic). To take an example of OP descent that you'd relate to: sistit is 3rd person sg. in Latin (right?); in OP, any initial pIE s becomes h, and the s after i, u, r, or any velar consonant becomes š, per the Ruki rule (which also applies to Slavic and Albanian). Then the conjugation is slightly different, so instead of sistit, you get a(h)ištati, the a being an emphatic prefix and the h, again, being susceptible to deletion.