Output impedance of an oscillator
Matched impedance is only necessary in certain specific situations. Typically, when you have a very fast driver and you wish to maximize power transfer or minimize reflections (or both!). And in these cases, the driver typically has a very low output impedance (a few ohms) and a series resistor chosen so that the output impedance is 50Ω.

27MHz is fast, but not really fast. Use a standard scope probe set to 10x (so it's a 10MΩ/~10pF load) and you should be able to probe the signal just fine. This way, your scope loads the output as little as possible. At these frequencies, loop area is also important, so use a ground clip and make sure you have a good ground (probing technique is important when you want accurate scope traces!).

At 27MHz, the capacitive load of the oscilloscope probe will be the dominant impedance, which as W5VO points out will be around 500Ω. If you just want to look at levels/frequency, this won't be an issue (the oscillator will drive a 15pF load), but if you are debugging signal integrity issues, this capacitive loading will change your signal enough that passive probes are completely out of the question; an active probe is necessary for these situations.

Finally, if you decide that you need a load, use a load the simulates the input impedance of the circuit that the oscillator will be driving. Typically, you would just ballpark it, and use, say, a 1kΩ/10kΩ resistor as a dummy load.