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All of the fish you have are shoaling fish that should be in larger groups. However, I would not fuss over the angel at this stage, as I assume it has been alone for a while and adding more can be disastrous, plus you need at least a 4-foot tank for a group of 5. So we'll leave the angel alone.
Otos should be in a group, but if the two are doing fine, OK.
The other fish though must have company. Corydoras are very social fish, and without more of their own they are under stress. Minimum 3 of one species, but 5 is better. You could have 6, with two species and 3 of each species if you like.
Neon Tetra, Cherry Barb, Harlequin Rasbora and Golden Danio (assuming this is the gold colour form of the Zebra Danio) are best in groups of minimum 6. We have fish profiles, second tab from the left in the blue bar across the top of the page; you can click on the fish name when it is shaded in posts, as here. Info on groups, minimum tanks, other care is included.
If you increase all these fish, you will need much more room than a 125 litre (33 gallon) tank.