THere are many ways to implement a filter. One RC is a first order passive filter, two cascaded is a second order passive filter. An op-amp wired to be a filter in the simplest way is a first-order active filter (and there are even multiple ways to implement the same filter on an op-amp, especially for more complicated filters like elliptical, butterworth, bessel, etc). Other implementations include switched capacitor and digital. All can be single order, or multi-order.
Passive filters (like just a single RC) are small and cheap but have impedance problems in that you have to know the impedance of the source and load and take them into account for it to function properly. This also includes cascading. With active filters you don't have to worry so much about the load (or filter) that comes before and after the current filter stage while in a passive filter you must take everything that is along the input path and output path.
Sure you can use an op-amp buffer with an passive filter to do this...but why not just wire an op-amp as an active filter?
If I cascade 7 first-order filters (properly) I have a 7th order filter. It is sometimes possible to wire up a single op-amp 2nd (or higher) order filter, but it's still the same thing if I cascade 3 first order filters + 2 second order filters. I still get a 7th order filter. THere's no difference between even or odd.
1st-1st-1st = 3rd order
2nd-2nd-2nd = 6th order
1st-8rd-2nd = 11th order
DOesn't matter.