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General basic knowledge that you probably already know but I have to say it - vosotros/vosotras is only really used in Spain, but it does show up in some literature

The vosotros of today is a bit different than historical vos, don't worry about it for now unless you're getting into older literature. What you need to know about historical vos is that it originally was the precursor to usted - where Vuestra Merced "Your Lordship/Ladyship/Grace" turned into usted over time. Essentially vos USED to be used for polite 2nd person "you", and the plural vosotros "you + others" literally was "you all"

Modern vosotros/vosotras in Spain is the plural of tú and so it's "you all" informal, whereas ustedes is "you all" plural

In Latin America for the most part, ustedes is used regardless of formality for plural

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Okay... so object pronoun os is used like te or les but it's used in every object pronoun situation

Direct Object Pronouns me -> yo te -> tú lo / la -> 3rd person singular los / las -> 3rd person plural nos -> nosotros, nosotras os -> vosotros, vosotras

Indirect Object Pronouns me -> yo te -> tú le -> 3rd person singular les -> 3rd person plural nos -> nosotros, nosotras os -> vosotros, vosotras

Reflexive / Pronomials me -> yo te -> tú se -> 3rd person, singular AND plural nos -> nosotros, nosotras os -> vosotros, vosotras

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For direct objects - the vosotros receives the action:

Os juzgo. = I'm judging you. Os castigaron. = They punished you. Os amo mucho. = I love you all so much. Os quiero mucho. = I love you all so much.

For indirect objects - vosotros is the recipient of an action, "to whom" or "for whom" something is done:

No os creo. = I don't believe you. Os están hablando. = They're talking to you. Tenemos que deciros algo. = We have something to tell you. Os tenemos que decir algo. = We have something to tell you.

For reflexives:

¿Os estáis concentrando? = Are you concentrating? ¿No os acordáis? = Don't you remember? Podéis imaginaros... = You can imagine... Os podéis imagnar... = You can imagine...

*imaginarse is more like "to put yourself in someone else's shoes", or "to have a good imagination/understanding"... it's like imagínate tú "picture it" or "just imagine"... except in vosotros instead of tú. You also say things like me imagino "I can imagine" - it's more supposition than imagination

In English we tend to qualify "imagine" to get that point across like "you can't even imagine" or "just imagine" reads as imaginarse to me without a doubt

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Note that in a reflexive, your subject and object are the same, so this is the one where you're going to have a vosotros construction

Also note for commands that are reflexive that os is used but stuck onto the verb but it is considered irregular because of the actual conjugation

marcha = leave [tú] márchate = go away [tú] marche = leave [usted] márchese = go away [usted] marchen = leave [plural / ustedes] márchense = go away [plural / ustedes] marchad = leave [vosotros] marchaos = go away [vosotros]

Note: Negative of marcharse for vosotros is no os marchéis "don't leave"

The weird one is ir vs irse where id "go" becomes idos "go away" for vosotros normal and vosotros reflexive

And the negative command is more normal (assuming you know command form of ir), since it's no os vayáis "don't leave"

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So in general as long as you know how to use specific object pronouns - direct, indirect, reflexives - you can use os very easily. The ones that will probably throw you off the most are reflexives because you have to know what's reflexive and how to conjugate vosotros whereas with direct and indirect objects, you're rarely using os with a vosotros

In other words: os digo "I'm telling you" vs. me decís "you all are telling me"