奇怪 (qí guài) is the standard, neutral word for 'strange, odd' — it describes something unusual or surprising. 怪 (guài) can also mean 'strange' but is more colloquial and has other meanings like 'to blame' or 'quite' (e.g., 怪好的 'quite good'). For a stronger, more literary sense of 'bizarre', use 离奇 (lí qí).