Welcome to AA!
I would not recommend adding an oscar period, and right now I think you should hold off on adding new fish entirely.
It is likely that your fish are hiding due to stress. You have some fish together that aren't really compatible in the long term. Your Mbuna (blue dolphin, electric yellow) are African Rift Lake cichlids and would prefer harder water than your other fish, and they have different dietary needs (more plant based foods) than your other fish.
Your silver sharks (likely bala sharks) get quite large, are easily startled, and would do better in larger groups. As adults, a group of bala sharks would need a larger tank than what you probably have. They might be ok for now, but IMO you should consider rehoming them eventually.
Your severums and firemouth are Central American cichlids. They are generally compatible; however, since you have 2 severums (these grow pretty large), I'd recommend a minimum of 75 gallons for these fish as adults.
Your jewel is a west African riverine cichlid. It is more similar in dietary needs and preferred water parameters to your Central American cichlids. Jewels can be quite aggressive. This may or may not work out in the long term.
Your clown loaches will eventually grow quite large, given proper care. It will take years, but these will eventually need a larger tank than what you probably have.
Do you know what kind of pleco you have? If it is a common pleco, these get absolutely huge and really should not even be sold IMHO, as the vast majority of hobbyists (myself included!) are not prepared to house these monsters in the long term.
I realize that most of the information in my post is not at all what you want to hear. If you are willing to re-work your stock list and plan the tank around either African rift lake cichlids OR Central American cichlids, I think we can help you figure something out where your fish won't hide so much.
