How to Express f(x,t) in Big Oh Notation?

[AxiomOfChoice]

Suppose I have a function of x and t such that

$$f(x,t) \leq x^{1/2} \sqrt{1+t^2}.$$

How should I express this in big Oh notation in terms of both x AND t?

 

[mfb]

$O(\sqrt{x}t)$?
That covers both parameters.