Yes, OX can be used without a speaker cabinet.
OX offers an amazing way to get a fully-saturated amplifier tone through your speaker at manageable volume levels, but it is also completely safe to use without a speaker cabinet connected at all.
OX will still properly load your amplifier, even without a speaker connected to OX. The line, SPDIF, and headphone outputs will all work fine as well without an actual cabinet present.
Note: To access the OX manual and affiliated documentation, click here.