If the oscar and severum are small yes, but if they are anywhere over a foot no.
These needlefish don't get that big, and they are relatively peaceful. Unless its got enough length and thickness on it the oscar will probably try to eat it. You could probably keep them together for a while, but be watchful because oscars grow faster than needlefish.
i had needle nose gars for 5 months and i never added salt? i even had them breed for a month straight. no salt. hatched all the eggs and still no salt? i agree, they would be a tasty meal for an oscar. my largest was only about 10-12 inches, but super thin.
We need a scientific name here! I've seen Ctenolucius hujeta, as "silver" needlenose gar. Or is it the belonid "gar" Xenentodon cancila? Very different in terms of care...