もしかしたらPerhaps, Maybe, Possibly

Both express possibility, but differ in how likely something seems. おそらく suggests a high chance and is often used with だろう or でしょう, e.g. おそらく来るでしょう 'Most likely, he will come.' もしかしたら suggests a lower chance or speculation, and pairs with かもしれない or かもしれません, e.g. もしかしたら雨が降るかもしれない 'Perhaps it will rain.'

あるいは and もしかしたら can overlap in one usage. あるいは is usually used mid-sentence to present alternatives, e.g. 月曜日あるいは火曜日 'Monday or Tuesday.' However, it can also appear at the start of a sentence to mean 'Perhaps,' e.g. あるいはそうかもしれない 'Perhaps that might be so.' もしかしたら can be used the same way, as in もしかしたらそうかもしれない, though あるいは carries a stronger nuance of weighing other possibilities.

かもしれない and もしかしたら both express possibility, but differ in placement and nuance. かもしれない appears at the end of sentences or phrases and means 'might' or 'may', e.g. 行くかもしれない ('I might go.'). もしかしたら comes at the beginning to emphasize the uncertainty, e.g. もしかしたら行くかもしれない ('Perhaps I might go.').