The fish will be fine in that tank for a little. Not forever, but save your money and invest in a larger tank for when the fish mature.
I believe the OP mention the 10 was a QT ?
if so the oscar and pleco should be fine for the duration of QT(4-6 weeks)
you will need to let the pleco grow(mature) to sex with any hope of getting it correct.
after that you will prob need to vent the plec to be certain.
the std 55 is 19" deep but for a fully mature Oscar and pleco, the 55 will be too small.
the Oscar will be better off in a 75..the common pleco ? and Oscar combo would benefit by an even larger tank.
a common pleco size can range from 11" to over 23"(common pleco is the dumping ground for pleco's not yet full identified.) as a result their size can vary a bit.
I'm pretty sure standard 55's are only a foot deep and 75's are 18".
We are confusing our terms here. the disputed measurement here is front to back, in which a standard 55g tank measures 12", and a 75g tank measures 18". top to bottom is the same, as is length.
the point to remember is that upgrading to a 55g is definitely a good thing to do and I applaud you for your efforts to accomodate the fish, but keep in mind that your oscar has the capability to outgrow a 55g in less than a year, and you won't notice stunting until it's already done.definitely upgrade ASAP to what you can, but ultimately look into something 75g or larger, to accomodate the minimum adult size of an oscar.
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I'm pretty sure standard 55's are only a foot deep and 75's are 18".