From writing as I am assuming OP is young or young"er". This all reeks of inexperience and a lack of appropriate knowledge of what to research. Go through this forum and there are a multitude of threads and posts about jags in tanks way smaller than 400gal. Some even packed 400gal tanks.
Growing out an oscar to put in a 400gal pond is a responsible thing to do given the pond was already running and not, as is often the case, "I will gety it when I need it."
The immediate blasting of OP who maybe did research but was inadequate, did not validate the raging.
I use this example below to show the more metered responses to a poster that OBVIOUSLY knew SFA and did SFA research yet was treated with the respect that has kept me on this forum for almost 2 decades.

OP: you made a mistake. Learn from it and try to better your research, or ask better questions from a super knowledgeable forum.
MFK ppl: Don't become that forum that blasts people willy-nilly. I have left countless forums, many of which don't exist any more, due to these keyboard warriors.
A reply like Duanes, explains WHY OP is ill-informed. Not just that he is wrong and dumb. Come on ppl, as a collective forum we are better than this. If OP had posed the question: Can a jag and an oscar live in a 400gal pond? Who, in their right mind would have said no. So let's not act like this youngster threw an oscar under bus and watched it drive over it. Many people here post about adding new fish to their ponds etc.
MFK vets are easy to spot in their answers that lack the dripping disdain of internet enlightened MFK people. And I believe these people mean well, but I fear, the forum will start taking after the internet in general if we do not curate what we are saying and assessing whether our answers are us taking a jab or are constructive criticism.
Let's be a community where new keen members can answer the mundane rehashed questions while ppl with actual experience lurk and correct where need be or offer anecdotes to help a situation or poster.
Growing out an oscar to put in a 400gal pond is a responsible thing to do given the pond was already running and not, as is often the case, "I will gety it when I need it."
The immediate blasting of OP who maybe did research but was inadequate, did not validate the raging.
I use this example below to show the more metered responses to a poster that OBVIOUSLY knew SFA and did SFA research yet was treated with the respect that has kept me on this forum for almost 2 decades.

OP: you made a mistake. Learn from it and try to better your research, or ask better questions from a super knowledgeable forum.
MFK ppl: Don't become that forum that blasts people willy-nilly. I have left countless forums, many of which don't exist any more, due to these keyboard warriors.
A reply like Duanes, explains WHY OP is ill-informed. Not just that he is wrong and dumb. Come on ppl, as a collective forum we are better than this. If OP had posed the question: Can a jag and an oscar live in a 400gal pond? Who, in their right mind would have said no. So let's not act like this youngster threw an oscar under bus and watched it drive over it. Many people here post about adding new fish to their ponds etc.
MFK vets are easy to spot in their answers that lack the dripping disdain of internet enlightened MFK people. And I believe these people mean well, but I fear, the forum will start taking after the internet in general if we do not curate what we are saying and assessing whether our answers are us taking a jab or are constructive criticism.
Let's be a community where new keen members can answer the mundane rehashed questions while ppl with actual experience lurk and correct where need be or offer anecdotes to help a situation or poster.